diff --git a/src/Init/System/ST.lean b/src/Init/System/ST.lean
index 0930afb427..6db6e016bf 100644
--- a/src/Init/System/ST.lean
+++ b/src/Init/System/ST.lean
@@ -21,12 +21,12 @@ class STWorld (σ : outParam Type) (m : Type → Type)
 instance STWorld.trans {σ m n} [STWorld σ m] [MonadLift m n] : STWorld σ n := ⟨⟩
 instance STWorld.base {ε σ} : STWorld σ (EST ε σ) := ⟨⟩
 
-def runEST {ε α : Type} (x : forall (σ : Type), EST ε σ α) : Except ε α :=
+@[inline] def runEST {ε α : Type} (x : forall (σ : Type), EST ε σ α) : Except ε α :=
 match x Unit () with
 | EStateM.Result.ok a _     => Except.ok a
 | EStateM.Result.error ex _ => Except.error ex
 
-def runST {α : Type} (x : forall (σ : Type), ST σ α) : α :=
+@[inline] def runST {α : Type} (x : forall (σ : Type), ST σ α) : α :=
 match x Unit () with
 | EStateM.Result.ok a _     => a
 | EStateM.Result.error ex _ => Empty.rec _ ex