diff --git a/src/proto_alpha/lib_protocol/sc_rollup_costs.ml b/src/proto_alpha/lib_protocol/sc_rollup_costs.ml
index 570a42142116aa9fcc6a9edf3280b0aae3b5b798..88d2f3facd5d5247b8368b37e222f793d5dd8e30 100644
--- a/src/proto_alpha/lib_protocol/sc_rollup_costs.ml
+++ b/src/proto_alpha/lib_protocol/sc_rollup_costs.ml
@@ -55,12 +55,12 @@ end
 
 (* We assume that the gas cost of adding messages [[ m_1; ... ; m_n]] at level
    [l] is linear in the sum of lengths of the messages, and it is logarithmic
-   in [l]. That is, [cost_add_external_messages([m_1; .. ; m_n], l)] =
+   in [l]. That is, [cost_add_serialized_messages([m_1; .. ; m_n], l)] =
    `n * cost_add_message_base +
     cost_add_message_per_bytes * \sum_{i=1}^n length(m_i) +
     cost_add_inbox_per_level * l`.
 *)
-let cost_add_external_messages ~num_messages ~total_messages_size l =
+let cost_add_serialized_messages ~num_messages ~total_messages_size l =
   let open S_syntax in
   let log_level =
     if Int32.equal l Int32.zero then Saturation_repr.safe_int 0
diff --git a/src/proto_alpha/lib_protocol/sc_rollup_costs.mli b/src/proto_alpha/lib_protocol/sc_rollup_costs.mli
index 3e75d0e43a1fb4ba96bc5a693c26540d449a3907..72d850c0d6405b76d786c2faba61b0ab541b5550 100644
--- a/src/proto_alpha/lib_protocol/sc_rollup_costs.mli
+++ b/src/proto_alpha/lib_protocol/sc_rollup_costs.mli
@@ -41,11 +41,11 @@ end
 val is_valid_parameters_ty_cost :
   ty_size:'a Saturation_repr.t -> Saturation_repr.may_saturate Saturation_repr.t
 
-(** [cost_add_external_messages ~num_messages ~total_messages_length level]
+(** [cost_add_serialized_messages ~num_messages ~total_messages_length level]
     returns the cost of adding [num_messages] with total messages size
     [total_messages_size] to a sc-rollup inbox at level [level]. This
     function is used internally in the [Sc_rollup_storage] module. *)
-val cost_add_external_messages :
+val cost_add_serialized_messages :
   num_messages:int -> total_messages_size:int -> int32 -> Gas_limit_repr.cost
 
 (** [cost_serialize_internal_inbox_message internal_inbox_message] is the cost
diff --git a/src/proto_alpha/lib_protocol/sc_rollup_inbox_storage.ml b/src/proto_alpha/lib_protocol/sc_rollup_inbox_storage.ml
index 0e20cfb8c211821b2ee7a6e0fa28877fc3e4babe..3b21d84ed353bdb06f7bef5c053ba332e2f143d7 100644
--- a/src/proto_alpha/lib_protocol/sc_rollup_inbox_storage.ml
+++ b/src/proto_alpha/lib_protocol/sc_rollup_inbox_storage.ml
@@ -119,13 +119,13 @@ let add_messages ctxt rollup messages =
   let*? current_messages, ctxt =
     Sc_rollup_in_memory_inbox.current_messages ctxt rollup
   in
-  let gas_cost_add_external_messages =
-    Sc_rollup_costs.cost_add_external_messages
+  let cost_add_serialized_messages =
+    Sc_rollup_costs.cost_add_serialized_messages
       ~num_messages
       ~total_messages_size
       levels
   in
-  let*? ctxt = Raw_context.consume_gas ctxt gas_cost_add_external_messages in
+  let*? ctxt = Raw_context.consume_gas ctxt cost_add_serialized_messages in
   (*
       Notice that the protocol is forgetful: it throws away the inbox
       history. On the contrary, the history is stored by the rollup
@@ -153,12 +153,17 @@ let add_external_messages ctxt rollup external_messages =
 
 let add_internal_message ctxt rollup ~payload ~sender ~source =
   let open Lwt_result_syntax in
-  (* TODO: #2951
-     Charge gas for serializing the internal message.
-  *)
+  let internal_message =
+    {Sc_rollup_inbox_message_repr.payload; sender; source}
+  in
+  (* Pay gas for serializing an internal message. *)
+  let*? ctxt =
+    Raw_context.consume_gas
+      ctxt
+      (Sc_rollup_costs.cost_serialize_internal_inbox_message internal_message)
+  in
   let*? message =
-    Sc_rollup_inbox_message_repr.(
-      to_bytes @@ Internal {payload; sender; source})
+    Sc_rollup_inbox_message_repr.(to_bytes @@ Internal internal_message)
   in
   add_messages ctxt rollup [message]