diff --git a/src/client/embedded/bootstrap/client_proto_rpcs.ml b/src/client/embedded/bootstrap/client_proto_rpcs.ml
index 4ca1c9c75..d3ac4ae3c 100644
--- a/src/client/embedded/bootstrap/client_proto_rpcs.ml
+++ b/src/client/embedded/bootstrap/client_proto_rpcs.ml
@@ -46,8 +46,8 @@ module Constants = struct
     call_error_service1 cctxt Services.Constants.voting_period_length block ()
   let time_before_reward cctxt block =
     call_error_service1 cctxt Services.Constants.time_before_reward block ()
-  let time_between_slots cctxt block =
-    call_error_service1 cctxt Services.Constants.time_between_slots block ()
+  let slot_durations cctxt block =
+    call_error_service1 cctxt Services.Constants.slot_durations block ()
   let first_free_mining_slot cctxt block =
     call_error_service1 cctxt Services.Constants.first_free_mining_slot block ()
   let max_signing_slot cctxt block =
diff --git a/src/client/embedded/bootstrap/client_proto_rpcs.mli b/src/client/embedded/bootstrap/client_proto_rpcs.mli
index 9bb26d016..8d9f82f62 100644
--- a/src/client/embedded/bootstrap/client_proto_rpcs.mli
+++ b/src/client/embedded/bootstrap/client_proto_rpcs.mli
@@ -35,9 +35,9 @@ module Constants : sig
   val time_before_reward:
     Client_commands.context ->
     block -> Period.t tzresult Lwt.t
-  val time_between_slots:
+  val slot_durations:
     Client_commands.context ->
-    block -> Period.t tzresult Lwt.t
+    block -> (Period.t list) tzresult Lwt.t
   val first_free_mining_slot:
     Client_commands.context ->
     block -> int32 tzresult Lwt.t
diff --git a/src/proto/bootstrap/constants_repr.ml b/src/proto/bootstrap/constants_repr.ml
index 6bcb6e9e4..62506c197 100644
--- a/src/proto/bootstrap/constants_repr.ml
+++ b/src/proto/bootstrap/constants_repr.ml
@@ -35,7 +35,7 @@ type constants = {
   cycle_length: int32 ;
   voting_period_length: int32 ;
   time_before_reward: Period_repr.t ;
-  time_between_slots: Period_repr.t ;
+  slot_durations: Period_repr.t list ;
   first_free_mining_slot: int32 ;
   max_signing_slot: int ;
   instructions_per_transaction: int ;
@@ -49,10 +49,8 @@ let default = {
     Period_repr.of_seconds_exn
     (* One year in seconds *)
     Int64.(mul 365L (mul 24L 3600L)) ;
-  time_between_slots =
-    Period_repr.of_seconds_exn
-      (* One minute in seconds *)
-      10L ;
+  slot_durations =
+    List.map Period_repr.of_seconds_exn [ 60L ] ;
   first_free_mining_slot = 16l ;
   max_signing_slot = 15 ;
   instructions_per_transaction = 16 * 1024 ;
@@ -71,38 +69,37 @@ let constants_encoding =
   (* let open Data_encoding in *)
   Data_encoding.conv
     (fun c ->
-       let open Compare in
+       let module Compare_slot_durations = Compare.List (Period_repr) in
        let cycle_length =
-         opt Int32.(=)
+         opt Compare.Int32.(=)
            default.cycle_length c.cycle_length
        and voting_period_length =
-         opt Int32.(=)
+         opt Compare.Int32.(=)
            default.voting_period_length c.voting_period_length
        and time_before_reward =
          map_option Period_repr.to_seconds @@
          opt Period_repr.(=)
            default.time_before_reward c.time_before_reward
-       and time_between_slots =
-         map_option Period_repr.to_seconds @@
-         opt Period_repr.(=)
-           default.time_between_slots c.time_between_slots
+       and slot_durations =
+         opt Compare_slot_durations.(=)
+           default.slot_durations c.slot_durations
        and first_free_mining_slot =
-         opt Int32.(=)
+         opt Compare.Int32.(=)
            default.first_free_mining_slot c.first_free_mining_slot
        and max_signing_slot =
-         opt Int.(=)
+         opt Compare.Int.(=)
            default.max_signing_slot c.max_signing_slot
        and instructions_per_transaction =
-         opt Int.(=)
+         opt Compare.Int.(=)
            default.instructions_per_transaction c.instructions_per_transaction
        and proof_of_work_threshold =
-         opt Int64.(=)
+         opt Compare.Int64.(=)
            default.proof_of_work_threshold c.proof_of_work_threshold
        in
        ( cycle_length,
          voting_period_length,
          time_before_reward,
-         time_between_slots,
+         slot_durations,
          first_free_mining_slot,
          max_signing_slot,
          instructions_per_transaction,
@@ -111,7 +108,7 @@ let constants_encoding =
     (fun ( cycle_length,
            voting_period_length,
            time_before_reward,
-           time_between_slots,
+           slot_durations,
            first_free_mining_slot,
            max_signing_slot,
            instructions_per_transaction,
@@ -124,9 +121,9 @@ let constants_encoding =
         time_before_reward =
           unopt default.time_before_reward @@
           map_option Period_repr.of_seconds_exn time_before_reward ;
-        time_between_slots =
-          unopt default.time_between_slots @@
-          map_option Period_repr.of_seconds_exn time_between_slots ;
+        slot_durations =
+          unopt default.slot_durations @@
+          slot_durations ;
         first_free_mining_slot =
           unopt default.first_free_mining_slot first_free_mining_slot ;
         max_signing_slot =
@@ -141,7 +138,7 @@ let constants_encoding =
         (opt "cycle_length" int32)
         (opt "voting_period_length" int32)
         (opt "time_before_reward" int64)
-        (opt "time_between_slots" int64)
+        (opt "slot_durations" (list Period_repr.encoding))
         (opt "first_free_mining_slot" int32)
         (opt "max_signing_slot" int31)
         (opt "instructions_per_transaction" int31)
diff --git a/src/proto/bootstrap/mining.ml b/src/proto/bootstrap/mining.ml
index 718d909d3..1815764da 100644
--- a/src/proto/bootstrap/mining.ml
+++ b/src/proto/bootstrap/mining.ml
@@ -17,13 +17,25 @@ type error +=
   | Cannot_pay_endorsement_bond
   | Bad_slot
   | Bad_delegate
+  | Invalid_slot_durations_constant
 
 let minimal_time c priority =
   Timestamp.get_current c >>=? fun prev_timestamp ->
+  let rec cumsum_slot_durations acc durations p =
+    if Compare.Int32.(=) p 0l then
+      ok acc
+    else match durations with
+      | [] -> Error_monad.error Invalid_slot_durations_constant
+      | [ last ] ->
+         Period.mult p last >>? fun period ->
+         Timestamp.(acc +? period)
+      | first :: durations ->
+         Timestamp.(acc +? first) >>? fun acc ->
+         let p = Int32.pred p in
+         cumsum_slot_durations acc durations p in
   Lwt.return
-    (Period.mult (Int32.succ priority)
-       (Constants.time_between_slots c)) >>=? fun period ->
-  Lwt.return Timestamp.(prev_timestamp +? period)
+    (cumsum_slot_durations
+       prev_timestamp (Constants.slot_durations c) priority)
 
 let check_timestamp c priority timestamp =
   minimal_time c priority >>=? fun minimal_time ->
diff --git a/src/proto/bootstrap/services.ml b/src/proto/bootstrap/services.ml
index a140f989d..ef1e158e7 100644
--- a/src/proto/bootstrap/services.ml
+++ b/src/proto/bootstrap/services.ml
@@ -60,12 +60,12 @@ module Constants = struct
                 describe ~title: "time before reward" Period.encoding)
       RPC.Path.(custom_root / "constants" / "time_before_reward")
 
-  let time_between_slots custom_root =
+  let slot_durations custom_root =
     RPC.service
-      ~description: "Time between slots"
+      ~description: "Slot durations"
       ~input: empty
       ~output: (wrap_tzerror @@
-                describe ~title: "time between slots" Period.encoding)
+                describe ~title: "time between slots" (list Period.encoding))
       RPC.Path.(custom_root / "constants" / "time_between_slots")
 
   let first_free_mining_slot custom_root =
diff --git a/src/proto/bootstrap/services_registration.ml b/src/proto/bootstrap/services_registration.ml
index 1328f2399..2231435db 100644
--- a/src/proto/bootstrap/services_registration.ml
+++ b/src/proto/bootstrap/services_registration.ml
@@ -53,10 +53,10 @@ let time_before_reward ctxt =
 
 let () = register0 Services.Constants.time_before_reward time_before_reward
 
-let time_between_slots ctxt =
-  return @@ Constants.time_between_slots ctxt
+let slot_durations ctxt =
+  return @@ Constants.slot_durations ctxt
 
-let () = register0 Services.Constants.time_between_slots time_between_slots
+let () = register0 Services.Constants.slot_durations slot_durations
 
 let first_free_mining_slot ctxt =
   return @@ Constants.first_free_mining_slot ctxt
diff --git a/src/proto/bootstrap/tezos_context.ml b/src/proto/bootstrap/tezos_context.ml
index d1c9287fd..98971134c 100644
--- a/src/proto/bootstrap/tezos_context.ml
+++ b/src/proto/bootstrap/tezos_context.ml
@@ -56,9 +56,9 @@ module Constants = struct
   let time_before_reward c =
     let constants = Storage.constants c in
     constants.time_before_reward
-  let time_between_slots c =
+  let slot_durations c =
     let constants = Storage.constants c in
-    constants.time_between_slots
+    constants.slot_durations
   let first_free_mining_slot c =
     let constants = Storage.constants c in
     constants.first_free_mining_slot
diff --git a/src/proto/bootstrap/tezos_context.mli b/src/proto/bootstrap/tezos_context.mli
index 88d3afc55..652b83566 100644
--- a/src/proto/bootstrap/tezos_context.mli
+++ b/src/proto/bootstrap/tezos_context.mli
@@ -181,7 +181,7 @@ module Constants : sig
   val cycle_length: context -> int32
   val voting_period_length: context -> int32
   val time_before_reward: context -> Period.t
-  val time_between_slots: context -> Period.t
+  val slot_durations: context -> Period.t list
   val first_free_mining_slot: context -> int32
   val max_signing_slot: context -> int
   val instructions_per_transaction: context -> int
diff --git a/test/sandbox.json b/test/sandbox.json
index cd431ad24..3a10b0c66 100644
--- a/test/sandbox.json
+++ b/test/sandbox.json
@@ -1,5 +1,5 @@
 {
-    "time_between_slots" : 10,
+    "slot_durations" : [ 10 ; 5 ],
     "cycle_length" : 128,
     "first_free_mining_slot" : 4
 }