diff --git a/Include/internal/pycore_pystate.h b/Include/internal/pycore_pystate.h
index c82e8db9051..d96ba312070 100644
--- a/Include/internal/pycore_pystate.h
+++ b/Include/internal/pycore_pystate.h
@@ -49,8 +49,18 @@ _Py_ThreadCanHandlePendingCalls(void)
 /* Variable and macro for in-line access to current thread
    and interpreter state */
 
-static inline PyThreadState* _PyRuntimeState_GetThreadState(_PyRuntimeState *runtime) {
+#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
+PyAPI_FUNC(PyThreadState*) _PyThreadState_GetTSS(void);
+#endif
+
+static inline PyThreadState*
+_PyRuntimeState_GetThreadState(_PyRuntimeState *runtime)
+{
+#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
+    return _PyThreadState_GetTSS();
+#else
     return (PyThreadState*)_Py_atomic_load_relaxed(&runtime->gilstate.tstate_current);
+#endif
 }
 
 /* Get the current Python thread state.
@@ -62,8 +72,14 @@ static inline PyThreadState* _PyRuntimeState_GetThreadState(_PyRuntimeState *run
    The caller must hold the GIL.
 
    See also PyThreadState_Get() and PyThreadState_GET(). */
-static inline PyThreadState *_PyThreadState_GET(void) {
+static inline PyThreadState*
+_PyThreadState_GET(void)
+{
+#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
+    return _PyThreadState_GetTSS();
+#else
     return _PyRuntimeState_GetThreadState(&_PyRuntime);
+#endif
 }
 
 /* Redefine PyThreadState_GET() as an alias to _PyThreadState_GET() */
diff --git a/Python/ceval.c b/Python/ceval.c
index 0c08a76f7d1..b5854d34464 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -380,9 +380,13 @@ PyEval_AcquireThread(PyThreadState *tstate)
     take_gil(tstate);
 
     struct _gilstate_runtime_state *gilstate = &tstate->interp->runtime->gilstate;
+#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
+    (void)_PyThreadState_Swap(gilstate, tstate);
+#else
     if (_PyThreadState_Swap(gilstate, tstate) != NULL) {
         Py_FatalError("non-NULL old thread state");
     }
+#endif
 }
 
 void
@@ -443,7 +447,12 @@ PyThreadState *
 PyEval_SaveThread(void)
 {
     _PyRuntimeState *runtime = &_PyRuntime;
+#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
+    PyThreadState *old_tstate = _PyThreadState_GET();
+    PyThreadState *tstate = _PyThreadState_Swap(&runtime->gilstate, old_tstate);
+#else
     PyThreadState *tstate = _PyThreadState_Swap(&runtime->gilstate, NULL);
+#endif
     ensure_tstate_not_null(__func__, tstate);
 
     struct _ceval_runtime_state *ceval = &runtime->ceval;
@@ -866,9 +875,13 @@ eval_frame_handle_pending(PyThreadState *tstate)
 
         take_gil(tstate);
 
+#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
+        (void)_PyThreadState_Swap(&runtime->gilstate, tstate);
+#else
         if (_PyThreadState_Swap(&runtime->gilstate, tstate) != NULL) {
             Py_FatalError("orphan tstate");
         }
+#endif
     }
 
     /* Check for asynchronous exception. */
diff --git a/Python/pystate.c b/Python/pystate.c
index dd957500272..119fe31a84b 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -956,6 +956,14 @@ _PyThreadState_DeleteExcept(_PyRuntimeState *runtime, PyThreadState *tstate)
 }
 
 
+#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
+PyThreadState*
+_PyThreadState_GetTSS(void) {
+    return PyThread_tss_get(&_PyRuntime.gilstate.autoTSSkey);
+}
+#endif
+
+
 PyThreadState *
 _PyThreadState_UncheckedGet(void)
 {
@@ -975,7 +983,11 @@ PyThreadState_Get(void)
 PyThreadState *
 _PyThreadState_Swap(struct _gilstate_runtime_state *gilstate, PyThreadState *newts)
 {
+#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
+    PyThreadState *oldts = _PyThreadState_GetTSS();
+#else
     PyThreadState *oldts = _PyRuntimeGILState_GetThreadState(gilstate);
+#endif
 
     _PyRuntimeGILState_SetThreadState(gilstate, newts);
     /* It should not be possible for more than one thread state
@@ -993,6 +1005,9 @@ _PyThreadState_Swap(struct _gilstate_runtime_state *gilstate, PyThreadState *new
             Py_FatalError("Invalid thread state for this thread");
         errno = err;
     }
+#endif
+#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
+    PyThread_tss_set(&gilstate->autoTSSkey, newts);
 #endif
     return oldts;
 }
@@ -1363,7 +1378,9 @@ PyGILState_Ensure(void)
 
     /* Ensure that _PyEval_InitThreads() and _PyGILState_Init() have been
        called by Py_Initialize() */
+#ifndef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
     assert(_PyEval_ThreadsInitialized(runtime));
+#endif
     assert(gilstate->autoInterpreterState);
 
     PyThreadState *tcur = (PyThreadState *)PyThread_tss_get(&gilstate->autoTSSkey);