diff --git a/reactos/dll/ntdll/def/ntdll.pspec b/reactos/dll/ntdll/def/ntdll.pspec
index 2c5de04b8d1..e87b71c6bf1 100644
--- a/reactos/dll/ntdll/def/ntdll.pspec
+++ b/reactos/dll/ntdll/def/ntdll.pspec
@@ -419,7 +419,7 @@
 @ stdcall RtlAcquireSRWLockShared(ptr)
 @ stdcall RtlActivateActivationContext(long ptr ptr)
 //@ stdcall RtlActivateActivationContextEx
-@ stdcall RtlActivateActivationContextUnsafeFast(ptr ptr)
+@ fastcall RtlActivateActivationContextUnsafeFast(ptr ptr)
 @ stdcall RtlAddAccessAllowedAce(ptr long long ptr)
 @ stdcall RtlAddAccessAllowedAceEx(ptr long long long ptr)
 @ stdcall RtlAddAccessAllowedObjectAce(ptr long long long ptr ptr ptr)
@@ -532,7 +532,7 @@
 @ stdcall RtlCutoverTimeToSystemTime(ptr ptr ptr long)
 @ stdcall RtlDeNormalizeProcessParams(ptr)
 @ stdcall RtlDeactivateActivationContext(long long)
-@ stdcall RtlDeactivateActivationContextUnsafeFast(ptr)
+@ fastcall RtlDeactivateActivationContextUnsafeFast(ptr)
 //@ stdcall RtlDebugPrintTimes
 @ stdcall RtlDecodePointer(ptr)
 @ stdcall RtlDecodeSystemPointer(ptr) RtlEncodeSystemPointer
diff --git a/reactos/dll/ntdll/ldr/ldrinit.c b/reactos/dll/ntdll/ldr/ldrinit.c
index 5913304361a..a7a196fcaa3 100644
--- a/reactos/dll/ntdll/ldr/ldrinit.c
+++ b/reactos/dll/ntdll/ldr/ldrinit.c
@@ -616,8 +616,8 @@ LdrpRunInitializeRoutines(IN PCONTEXT Context OPTIONAL)
 
             /* Set up the Act Ctx */
             ActCtx.Size = sizeof(ActCtx);
-            ActCtx.Frame.Flags = ACTCTX_FLAG_PROCESSOR_ARCHITECTURE_VALID;
-            RtlZeroMemory(&ActCtx, sizeof(ActCtx));
+            ActCtx.Format = 1;
+            RtlZeroMemory(&ActCtx.Frame, sizeof(RTL_ACTIVATION_CONTEXT_STACK_FRAME));
 
             /* Activate the ActCtx */
             RtlActivateActivationContextUnsafeFast(&ActCtx,
@@ -682,8 +682,8 @@ LdrpRunInitializeRoutines(IN PCONTEXT Context OPTIONAL)
     {
         /* Set up the Act Ctx */
         ActCtx.Size = sizeof(ActCtx);
-        ActCtx.Frame.Flags = ACTCTX_FLAG_PROCESSOR_ARCHITECTURE_VALID;
-        RtlZeroMemory(&ActCtx, sizeof(ActCtx));
+        ActCtx.Format = 1;
+        RtlZeroMemory(&ActCtx.Frame, sizeof(RTL_ACTIVATION_CONTEXT_STACK_FRAME));
 
         /* Activate the ActCtx */
         RtlActivateActivationContextUnsafeFast(&ActCtx,
diff --git a/reactos/dll/ntdll/ldr/ldrpe.c b/reactos/dll/ntdll/ldr/ldrpe.c
index 730d260d836..12e24fe547d 100644
--- a/reactos/dll/ntdll/ldr/ldrpe.c
+++ b/reactos/dll/ntdll/ldr/ldrpe.c
@@ -660,8 +660,8 @@ LdrpWalkImportDescriptor(IN LPWSTR DllPath OPTIONAL,
 DPRINT1("LdrpWalkImportDescriptor('%S' %x)\n", DllPath, LdrEntry);
     /* Set up the Act Ctx */
     ActCtx.Size = sizeof(ActCtx);
-    ActCtx.Frame.Flags = ACTCTX_FLAG_PROCESSOR_ARCHITECTURE_VALID;
-    RtlZeroMemory(&ActCtx, sizeof(ActCtx));
+    ActCtx.Frame.Flags = 1;
+    RtlZeroMemory(&ActCtx.Frame, sizeof(RTL_ACTIVATION_CONTEXT_STACK_FRAME));
 
     /* Check if we have a manifest prober routine */
     if (LdrpManifestProberRoutine)
diff --git a/reactos/include/ndk/rtlfuncs.h b/reactos/include/ndk/rtlfuncs.h
index a5c453f2e32..6807128b626 100644
--- a/reactos/include/ndk/rtlfuncs.h
+++ b/reactos/include/ndk/rtlfuncs.h
@@ -3071,8 +3071,8 @@ RtlAddRefActivationContext(
 
 
 NTSYSAPI
-NTSTATUS
-NTAPI
+PRTL_ACTIVATION_CONTEXT_STACK_FRAME
+FASTCALL
 RtlActivateActivationContextUnsafeFast(
     IN PRTL_CALLER_ALLOCATED_ACTIVATION_CONTEXT_STACK_FRAME_EXTENDED Frame,
     IN PVOID Context
@@ -3121,8 +3121,8 @@ NTAPI
 RtlFreeThreadActivationContextStack(void);
 
 NTSYSAPI
-NTSTATUS
-NTAPI
+PRTL_ACTIVATION_CONTEXT_STACK_FRAME
+FASTCALL
 RtlDeactivateActivationContextUnsafeFast(
     IN PRTL_CALLER_ALLOCATED_ACTIVATION_CONTEXT_STACK_FRAME_EXTENDED Frame
 );
diff --git a/reactos/lib/rtl/actctx.c b/reactos/lib/rtl/actctx.c
index cd900e1b8de..1a6dc569178 100644
--- a/reactos/lib/rtl/actctx.c
+++ b/reactos/lib/rtl/actctx.c
@@ -22,6 +22,8 @@
 
 #include <wine/unicode.h>
 
+BOOLEAN RtlpNotAllowingMultipleActivation;
+
 #define QUERY_ACTCTX_FLAG_ACTIVE (0x00000001)
 
 #define ACTCTX_FLAGS_ALL (\
@@ -2730,31 +2732,75 @@ RtlAllocateActivationContextStack(IN PVOID *Context)
     return STATUS_SUCCESS;
 }
 
-NTSTATUS
-NTAPI
+PRTL_ACTIVATION_CONTEXT_STACK_FRAME
+FASTCALL
 RtlActivateActivationContextUnsafeFast(IN PRTL_CALLER_ALLOCATED_ACTIVATION_CONTEXT_STACK_FRAME_EXTENDED Frame,
                                        IN PVOID Context)
 {
-    static int i;
+#if NEW_NTDLL_LOADER
+    RTL_ACTIVATION_CONTEXT_STACK_FRAME *ActiveFrame;
 
-    if (i == 0)
-        UNIMPLEMENTED;
-    i++;
+    /* Get the curren active frame */
+    ActiveFrame = NtCurrentTeb()->ActivationContextStackPointer->ActiveFrame;
 
-    return STATUS_NOT_IMPLEMENTED;
+    DPRINT1("ActiveFrame %p, &Frame->Frame %p, Context %p\n", ActiveFrame, &Frame->Frame, Context);
+
+    /* Actually activate it */
+    Frame->Frame.Previous = ActiveFrame;
+    Frame->Frame.ActivationContext = Context;
+    Frame->Frame.Flags = 0;
+
+    /* Check if we can activate this context */
+    if ((ActiveFrame && (ActiveFrame->ActivationContext != Context)) ||
+        Context)
+    {
+        /* Set new active frame */
+        NtCurrentTeb()->ActivationContextStackPointer->ActiveFrame = &Frame->Frame;
+        return &Frame->Frame;
+    }
+
+    /* We can get here only one way: it was already activated */
+    DPRINT1("Trying to activate improper activation context\n");
+
+    /* Activate only if we are allowing multiple activation */
+    if (!RtlpNotAllowingMultipleActivation)
+    {
+        NtCurrentTeb()->ActivationContextStackPointer->ActiveFrame = &Frame->Frame;
+    }
+    else
+    {
+        /* Set flag */
+        Frame->Frame.Flags = 0x30;
+    }
+
+    /* Return pointer to the activation frame */
+    return &Frame->Frame;
+#else
+    return NULL;
+#endif
 }
 
-NTSTATUS
-NTAPI
+PRTL_ACTIVATION_CONTEXT_STACK_FRAME
+FASTCALL
 RtlDeactivateActivationContextUnsafeFast(IN PRTL_CALLER_ALLOCATED_ACTIVATION_CONTEXT_STACK_FRAME_EXTENDED Frame)
 {
-    static int i;
+#if NEW_NTDLL_LOADER
+    RTL_ACTIVATION_CONTEXT_STACK_FRAME *frame, *top;
 
-    if (i == 0)
-        UNIMPLEMENTED;
-    i++;
+    /* find the right frame */
+    top = NtCurrentTeb()->ActivationContextStackPointer->ActiveFrame;
+    frame = &Frame->Frame;
 
-    return STATUS_NOT_IMPLEMENTED;
+    if (!frame)
+        RtlRaiseStatus( STATUS_SXS_INVALID_DEACTIVATION );
+
+    /* pop everything up to and including frame */
+    NtCurrentTeb()->ActivationContextStackPointer->ActiveFrame = frame->Previous;
+
+    return frame;
+#else
+    return NULL;
+#endif
 }