diff --git a/Include/internal/pycore_ceval.h b/Include/internal/pycore_ceval.h
index 18c8f027af1..36899009908 100644
--- a/Include/internal/pycore_ceval.h
+++ b/Include/internal/pycore_ceval.h
@@ -50,7 +50,11 @@ extern PyObject *_PyEval_EvalCode(
     PyObject *kwdefs, PyObject *closure,
     PyObject *name, PyObject *qualname);
 
+#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
+extern int _PyEval_ThreadsInitialized(PyInterpreterState *interp);
+#else
 extern int _PyEval_ThreadsInitialized(struct pyruntimestate *runtime);
+#endif
 extern PyStatus _PyEval_InitGIL(PyThreadState *tstate);
 extern void _PyEval_FiniGIL(PyThreadState *tstate);
 
diff --git a/Include/internal/pycore_interp.h b/Include/internal/pycore_interp.h
index 5bf8998e673..26e7a473a12 100644
--- a/Include/internal/pycore_interp.h
+++ b/Include/internal/pycore_interp.h
@@ -46,6 +46,9 @@ struct _ceval_state {
     /* Request for dropping the GIL */
     _Py_atomic_int gil_drop_request;
     struct _pending_calls pending;
+#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
+    struct _gil_runtime_state gil;
+#endif
 };
 
 
diff --git a/Include/internal/pycore_runtime.h b/Include/internal/pycore_runtime.h
index 34eb492b9f2..ebdc12b23a9 100644
--- a/Include/internal/pycore_runtime.h
+++ b/Include/internal/pycore_runtime.h
@@ -19,7 +19,9 @@ struct _ceval_runtime_state {
        the main thread of the main interpreter can handle signals: see
        _Py_ThreadCanHandleSignals(). */
     _Py_atomic_int signals_pending;
+#ifndef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
     struct _gil_runtime_state gil;
+#endif
 };
 
 /* GIL state */
diff --git a/Python/ceval.c b/Python/ceval.c
index b5854d34464..6435bd05446 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -250,6 +250,21 @@ ensure_tstate_not_null(const char *func, PyThreadState *tstate)
 }
 
 
+#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
+int
+_PyEval_ThreadsInitialized(PyInterpreterState *interp)
+{
+    return gil_created(&interp->ceval.gil);
+}
+
+int
+PyEval_ThreadsInitialized(void)
+{
+    // Fatal error if there is no current interpreter
+    PyInterpreterState *interp = PyInterpreterState_Get();
+    return _PyEval_ThreadsInitialized(interp);
+}
+#else
 int
 _PyEval_ThreadsInitialized(_PyRuntimeState *runtime)
 {
@@ -262,18 +277,25 @@ PyEval_ThreadsInitialized(void)
     _PyRuntimeState *runtime = &_PyRuntime;
     return _PyEval_ThreadsInitialized(runtime);
 }
+#endif
 
 PyStatus
 _PyEval_InitGIL(PyThreadState *tstate)
 {
+#ifndef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
     if (!_Py_IsMainInterpreter(tstate)) {
         /* Currently, the GIL is shared by all interpreters,
            and only the main interpreter is responsible to create
            and destroy it. */
         return _PyStatus_OK();
     }
+#endif
 
+#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
+    struct _gil_runtime_state *gil = &tstate->interp->ceval.gil;
+#else
     struct _gil_runtime_state *gil = &tstate->interp->runtime->ceval.gil;
+#endif
     assert(!gil_created(gil));
 
     PyThread_init_thread();
@@ -288,14 +310,20 @@ _PyEval_InitGIL(PyThreadState *tstate)
 void
 _PyEval_FiniGIL(PyThreadState *tstate)
 {
+#ifndef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
     if (!_Py_IsMainInterpreter(tstate)) {
         /* Currently, the GIL is shared by all interpreters,
            and only the main interpreter is responsible to create
            and destroy it. */
         return;
     }
+#endif
 
+#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
+    struct _gil_runtime_state *gil = &tstate->interp->ceval.gil;
+#else
     struct _gil_runtime_state *gil = &tstate->interp->runtime->ceval.gil;
+#endif
     if (!gil_created(gil)) {
         /* First Py_InitializeFromConfig() call: the GIL doesn't exist
            yet: do nothing. */
@@ -413,13 +441,18 @@ PyEval_ReleaseThread(PyThreadState *tstate)
 void
 _PyEval_ReInitThreads(_PyRuntimeState *runtime)
 {
+    PyThreadState *tstate = _PyRuntimeState_GetThreadState(runtime);
+    ensure_tstate_not_null(__func__, tstate);
+
+#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
+    struct _gil_runtime_state *gil = &tstate->interp->ceval.gil;
+#else
     struct _gil_runtime_state *gil = &runtime->ceval.gil;
+#endif
     if (!gil_created(gil)) {
         return;
     }
     recreate_gil(gil);
-    PyThreadState *tstate = _PyRuntimeState_GetThreadState(runtime);
-    ensure_tstate_not_null(__func__, tstate);
 
     take_gil(tstate);
 
@@ -457,7 +490,11 @@ PyEval_SaveThread(void)
 
     struct _ceval_runtime_state *ceval = &runtime->ceval;
     struct _ceval_state *ceval2 = &tstate->interp->ceval;
+#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
+    assert(gil_created(&ceval2->gil));
+#else
     assert(gil_created(&ceval->gil));
+#endif
     drop_gil(ceval, ceval2, tstate);
     return tstate;
 }
@@ -716,7 +753,9 @@ void
 _PyEval_InitRuntimeState(struct _ceval_runtime_state *ceval)
 {
     _Py_CheckRecursionLimit = Py_DEFAULT_RECURSION_LIMIT;
+#ifndef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
     _gil_initialize(&ceval->gil);
+#endif
 }
 
 int
@@ -731,6 +770,11 @@ _PyEval_InitState(struct _ceval_state *ceval)
     if (pending->lock == NULL) {
         return -1;
     }
+
+#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
+    _gil_initialize(&ceval->gil);
+#endif
+
     return 0;
 }
 
diff --git a/Python/ceval_gil.h b/Python/ceval_gil.h
index f25f8100732..56944b89237 100644
--- a/Python/ceval_gil.h
+++ b/Python/ceval_gil.h
@@ -144,7 +144,11 @@ static void
 drop_gil(struct _ceval_runtime_state *ceval, struct _ceval_state *ceval2,
          PyThreadState *tstate)
 {
+#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
+    struct _gil_runtime_state *gil = &ceval2->gil;
+#else
     struct _gil_runtime_state *gil = &ceval->gil;
+#endif
     if (!_Py_atomic_load_relaxed(&gil->locked)) {
         Py_FatalError("drop_gil: GIL is not locked");
     }
@@ -228,7 +232,11 @@ take_gil(PyThreadState *tstate)
     PyInterpreterState *interp = tstate->interp;
     struct _ceval_runtime_state *ceval = &interp->runtime->ceval;
     struct _ceval_state *ceval2 = &interp->ceval;
+#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
+    struct _gil_runtime_state *gil = &ceval2->gil;
+#else
     struct _gil_runtime_state *gil = &ceval->gil;
+#endif
 
     /* Check that _PyEval_InitThreads() was called to create the lock */
     assert(gil_created(gil));
@@ -320,10 +328,22 @@ _ready:
 
 void _PyEval_SetSwitchInterval(unsigned long microseconds)
 {
-    _PyRuntime.ceval.gil.interval = microseconds;
+#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
+    PyInterpreterState *interp = PyInterpreterState_Get();
+    struct _gil_runtime_state *gil = &interp->ceval.gil;
+#else
+    struct _gil_runtime_state *gil = &_PyRuntime.ceval.gil;
+#endif
+    gil->interval = microseconds;
 }
 
 unsigned long _PyEval_GetSwitchInterval()
 {
-    return _PyRuntime.ceval.gil.interval;
+#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
+    PyInterpreterState *interp = PyInterpreterState_Get();
+    struct _gil_runtime_state *gil = &interp->ceval.gil;
+#else
+    struct _gil_runtime_state *gil = &_PyRuntime.ceval.gil;
+#endif
+    return gil->interval;
 }
diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c
index 2149d8928d5..da66a82ada7 100644
--- a/Python/pylifecycle.c
+++ b/Python/pylifecycle.c
@@ -1561,9 +1561,13 @@ new_interpreter(PyThreadState **tstate_p, int isolated_subinterpreter)
 
     /* Copy the current interpreter config into the new interpreter */
     const PyConfig *config;
+#ifndef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
     if (save_tstate != NULL) {
         config = _PyInterpreterState_GetConfig(save_tstate->interp);
-    } else {
+    }
+    else
+#endif
+    {
         /* No current thread state, copy from the main interpreter */
         PyInterpreterState *main_interp = PyInterpreterState_Main();
         config = _PyInterpreterState_GetConfig(main_interp);