diff --git a/mps/src/event.c b/mps/src/event.c
index 1b53e017a95..02f72fceb49 100644
--- a/mps/src/event.c
+++ b/mps/src/event.c
@@ -1,6 +1,6 @@
 /* impl.c.event: EVENT LOGGING
  *
- * $HopeName: MMsrc!event.c(trunk.4) $
+ * $HopeName: MMsrc!event.c(trunk.5) $
  * Copyright (C) 1997 Harlequin Group, all rights reserved.
  *
  * .readership: MPS developers.
@@ -26,7 +26,7 @@
 #include "event.h"
 #include "mpsio.h"
 
-SRCID(event, "$HopeName: MMsrc!event.c(trunk.4) $");
+SRCID(event, "$HopeName: MMsrc!event.c(trunk.5) $");
 
 #ifdef EVENT /* .trans.ifdef */
 
@@ -34,11 +34,10 @@ static Bool eventInited = FALSE;
 static mps_io_t eventIO;
 static char eventBuffer[EVENT_BUFFER_SIZE];
 static Count eventUserCount;
-static Word EventKindControlBuffer[BTSize(EventKindNumber)]; 
 
 EventUnion Event; /* Used by macros in impl.h.event */
 char *EventNext, *EventLimit; /* Used by macros in impl.h.event */
-BT EventKindControl; /* Used to control output. */
+Word EventKindControl; /* Bit set used to control output. */
 
 Res EventFlush(void)
 {
@@ -67,10 +66,9 @@ Res (EventInit)(void)
     if(res != ResOK) return res;
     EventNext = eventBuffer;
     EventLimit = &eventBuffer[EVENT_BUFFER_SIZE];
-    eventUserCount = 0;
+    eventUserCount = (Count)0;
     eventInited = TRUE;
-    EventKindControl = (BT)EventKindControlBuffer;
-    BTSetRange(EventKindControl, 0, EventKindNumber);
+    EventKindControl = (Word)mps_lib_telemetry_control();
   }
 
   ++eventUserCount;
@@ -89,6 +87,30 @@ void (EventFinish)(void)
   --eventUserCount;
 }
 
+/* EventControl -- Change or read control word
+ *
+ * Resets the bits specified in resetMask, and flips those in
+ * flipMask.  Returns old value.
+ *
+ * Operations can be implemented as follows:
+ *   Set(M)   EventControl(M,M)
+ *   Reset(M) EventControl(M,0)
+ *   Flip(M)  EventControl(0,M)
+ *   Read()   EventControl(0,0)
+ */
+   
+Word EventControl(Word resetMask, Word flipMask)
+{
+  Word oldValue = EventKindControl;
+      
+  /* EventKindControl = (EventKindControl & ~resetMask) ^ flipMask */
+  EventKindControl =
+    BS_SYM_DIFF(BS_DIFF(EventKindControl, resetMask), flipMask);
+       
+  return oldValue;
+}
+
+
 #else /* EVENT, not */
 
 Res (EventInit)(void)
@@ -101,4 +123,9 @@ void (EventFinish)(void)
   NOOP;
 }
 
+Word EventControl(Word resetMask, Word flipMask)
+{
+  return (Word)0;
+}
+
 #endif /* EVENT */
diff --git a/mps/src/event.h b/mps/src/event.h
index 1cd5c35c008..1f9748f75d2 100644
--- a/mps/src/event.h
+++ b/mps/src/event.h
@@ -1,7 +1,7 @@
 /* impl.h.event -- Event Logging Interface
  *
  * Copyright (C) 1997 Harlequin Group, all rights reserved.
- * $HopeName: MMsrc!event.h(trunk.4) $
+ * $HopeName: MMsrc!event.h(trunk.5) $
  *
  * READERSHIP
  *
@@ -28,6 +28,7 @@
 extern Res EventFlush(void);
 extern Res EventInit(void);
 extern void EventFinish(void);
+extern Word EventControl(Word, Word);
 
 typedef Index EventKind;
 
@@ -96,7 +97,7 @@ extern EventUnion Event;
     Event.any.clock = mps_clock(); 
 
 #define EVENT_END(type, length) \
-  if(BTGet(EventKindControl, ((Index)Event ## type ## Kind))) { \
+  if(BS_IS_MEMBER(EventKindControl, ((Index)Event ## type ## Kind))) { \
     AVER(EventNext <= EventLimit); \
     if((length) > (size_t)(EventLimit - EventNext)) \
       EventFlush(); /* @@@ should pass length */ \
@@ -107,7 +108,7 @@ extern EventUnion Event;
   END
 
 extern char *EventNext, *EventLimit;
-extern BT EventKindControl;
+extern Word EventKindControl;
 
 #else /* EVENT not */
 
diff --git a/mps/src/misc.h b/mps/src/misc.h
index e5cdeda703d..b13696fd535 100644
--- a/mps/src/misc.h
+++ b/mps/src/misc.h
@@ -1,6 +1,6 @@
 /* impl.h.misc: MISCELLANEOUS DEFINITIONS
  *
- * $HopeName: MMsrc!misc.h(trunk.17) $
+ * $HopeName: MMsrc!misc.h(trunk.18) $
  * Copyright (C) 1997 The Harlequin Group Limited.  All rights reserved.
  *
  * Small general things which are useful for C but aren't part of the
@@ -129,6 +129,7 @@ typedef const struct SrcIdStruct {
 #define BS_SUPER(s1, s2)        (BS_INTER((s1), (s2)) == (s2))
 #define BS_SUB(s1, s2)          BS_SUPER((s2), (s1))
 #define BS_IS_SINGLE(s)         (((s) & ((s)-1)) == 0)
+#define BS_SYM_DIFF(s1, s2)     ((s1) ^ (s2))
 
 
 #endif /* misc_h */
diff --git a/mps/src/mps.h b/mps/src/mps.h
index 88eafbeffcf..b5498735fb5 100644
--- a/mps/src/mps.h
+++ b/mps/src/mps.h
@@ -1,6 +1,6 @@
 /* impl.h.mps: HARLEQUIN MEMORY POOL SYSTEM C INTERFACE
  *
- * $HopeName: MMsrc!mps.h(trunk.30) $
+ * $HopeName: MMsrc!mps.h(trunk.31) $
  * Copyright (C) 1997 The Harlequin Group Limited.  All rights reserved.
  *
  * .readership: customers, MPS developers.
@@ -364,6 +364,11 @@ extern mps_res_t mps_finalize(mps_arena_t, mps_addr_t *);
 extern void mps_definalize(mps_arena_t, mps_addr_t *);
 
 
+/* Telemetry Control */
+
+mps_word_t mps_telemetry_control(mps_word_t, mps_word_t);
+
+
 /* Scanner Support */
 
 extern mps_res_t mps_fix(mps_ss_t, mps_addr_t *);
diff --git a/mps/src/mpsi.c b/mps/src/mpsi.c
index e6eda125f4b..f41e62f602d 100644
--- a/mps/src/mpsi.c
+++ b/mps/src/mpsi.c
@@ -1,6 +1,6 @@
 /* impl.c.mpsi: MEMORY POOL SYSTEM C INTERFACE LAYER
  *
- * $HopeName: MMsrc!mpsi.c(trunk.39) $
+ * $HopeName: MMsrc!mpsi.c(trunk.40) $
  * Copyright (C) 1997 The Harlequin Group Limited.  All rights reserved.
  *
  * .purpose: This code bridges between the MPS interface to C,
@@ -52,7 +52,7 @@
 #include "mps.h"
 #include "mpsavm.h" /* only for mps_space_create */
 
-SRCID(mpsi, "$HopeName: MMsrc!mpsi.c(trunk.39) $");
+SRCID(mpsi, "$HopeName: MMsrc!mpsi.c(trunk.40) $");
 
 
 /* mpsi_check -- check consistency of interface mappings
@@ -1152,3 +1152,13 @@ void mps_message_finalization_ref(mps_addr_t *mps_addr_return,
 
   ArenaLeave(arena);
 }
+
+
+/* Telemetry control */
+
+mps_word_t mps_telemetry_control(mps_word_t resetMask, 
+                                 mps_word_t flipMask)
+{
+  /* Doesn't require locking and isn't arena-specific. */
+  return EventControl((Word)resetMask, (Word)flipMask);
+}
diff --git a/mps/src/mpslib.h b/mps/src/mpslib.h
index ae607369f0c..03dfd6dc9c0 100644
--- a/mps/src/mpslib.h
+++ b/mps/src/mpslib.h
@@ -1,6 +1,6 @@
 /* impl.h.mpslib: HARLEQUIN MEMORY POOL SYSTEM LIBRARY INTERFACE
  *
- * $HopeName: MMsrc!mpslib.h(trunk.4) $
+ * $HopeName: MMsrc!mpslib.h(trunk.5) $
  * Copyright (C) 1996,1997 Harlequin Group, all rights reserved.
  *
  * .readership: MPS client application developers, MPS developers.
@@ -45,4 +45,6 @@ extern void *mps_lib_memcpy(void *, const void *, size_t);
 typedef unsigned long mps_clock_t;
 extern mps_clock_t mps_clock(void);
 
+extern unsigned long mps_lib_telemetry_control(void);
+
 #endif /* mpslib_h */
diff --git a/mps/src/mpsliban.c b/mps/src/mpsliban.c
index 058dadda011..3793457fc8a 100644
--- a/mps/src/mpsliban.c
+++ b/mps/src/mpsliban.c
@@ -1,6 +1,6 @@
 /* impl.c.mpsliban: HARLEQUIN MEMORY POOL SYSTEM LIBRARY INTERFACE (ANSI)
  *
- * $HopeName: MMsrc!mpsliban.c(trunk.5) $
+ * $HopeName: MMsrc!mpsliban.c(trunk.6) $
  * Copyright (C) 1996,1997 Harlequin Group, all rights reserved.
  *
  * PURPOSE
@@ -89,3 +89,14 @@ mps_clock_t mps_clock(void)
 {
   return (unsigned long)clock();
 }
+
+unsigned long mps_lib_telemetry_control(void)
+{
+  char *s;
+
+  s = getenv("MPS_TELEMETRY_CONTROL");
+  if(s != NULL)
+    return strtoul(s, (char **)NULL, 0);
+  else
+    return 0;
+}