diff --git a/src/pkg/net/rpc/client.go b/src/pkg/net/rpc/client.go
index 4b0c9c3bba20efcfd4f8bc22d790e71cafa5c90b..608894fb0d6fe55eca35609a8570ffeea48f270f 100644
--- a/src/pkg/net/rpc/client.go
+++ b/src/pkg/net/rpc/client.go
@@ -58,6 +58,7 @@ type Client struct {
 // argument to force the body of the response to be read and then
 // discarded.
 type ClientCodec interface {
+	// WriteRequest must be safe for concurrent use by multiple goroutines.
 	WriteRequest(*Request, interface{}) error
 	ReadResponseHeader(*Response) error
 	ReadResponseBody(interface{}) error
diff --git a/src/pkg/net/rpc/server.go b/src/pkg/net/rpc/server.go
index fe1cdde1ad9e2a6cd9014d454e3b31fc5f7fb55d..9ccd703b12ebf5b81f0306f6c967d8733fb7962b 100644
--- a/src/pkg/net/rpc/server.go
+++ b/src/pkg/net/rpc/server.go
@@ -616,6 +616,7 @@ func RegisterName(name string, rcvr interface{}) error {
 type ServerCodec interface {
 	ReadRequestHeader(*Request) error
 	ReadRequestBody(interface{}) error
+	// WriteResponse must be safe for concurrent use by multiple goroutines.
 	WriteResponse(*Response, interface{}) error
 
 	Close() error