diff --git a/whalesong/js-assembler/runtime-src/baselib-exceptions.js b/whalesong/js-assembler/runtime-src/baselib-exceptions.js
index ec1030b..d12792e 100644
--- a/whalesong/js-assembler/runtime-src/baselib-exceptions.js
+++ b/whalesong/js-assembler/runtime-src/baselib-exceptions.js
@@ -80,6 +80,16 @@
     };
 
 
+    var raiseExnBreak = function(MACHINE, msg) {
+        var contMarks = MACHINE.captureContinuationMarks();
+        var k = false;
+        // FIXME: capture the current continuation and stuff it into
+        // k, to allow restart if possible.
+        raise(MACHINE, ExnBreak.constructor([msg, contMarks, k]));
+    };
+
+
+
     var raiseFailure = function(MACHINE, msg) {
         var contMarks = MACHINE.captureContinuationMarks();
         raise(MACHINE, ExnFail.constructor([msg, contMarks]));
@@ -208,7 +218,7 @@
     exceptions.exnSetContMarks = function(exn, v) { Exn.mutator(exn, 1, v); };
 
     exceptions.ExnBreak = ExnBreak;
-    exceptions.makeExnBreak = function(msg, marks) { return ExnBreak.constructor([msg, marks]); };
+    exceptions.makeExnBreak = function(msg, marks, k) { return ExnBreak.constructor([msg, marks, k]); };
     exceptions.isExnBreak = ExnBreak.predicate;
     exceptions.exnBreakContinuation = 
         function(exn) { return ExnBreak.accessor(exn, 0); };
@@ -244,6 +254,7 @@
 
 
     exceptions.raise = raise;
+    exceptions.raiseExnBreak = raiseExnBreak;
     exceptions.raiseFailure = raiseFailure;
     exceptions.raiseContractError = raiseContractError;
     exceptions.raiseDivisionByZeroError = raiseDivisionByZeroError;
@@ -258,4 +269,4 @@
     exceptions.raiseModuleLoadingError = raiseModuleLoadingError;
 
 
-}(this.plt.baselib));
\ No newline at end of file
+}(this.plt.baselib));
diff --git a/whalesong/js-assembler/runtime-src/baselib-functions.js b/whalesong/js-assembler/runtime-src/baselib-functions.js
index de51c18..b60fd8a 100644
--- a/whalesong/js-assembler/runtime-src/baselib-functions.js
+++ b/whalesong/js-assembler/runtime-src/baselib-functions.js
@@ -201,6 +201,16 @@
         }
 
         var i;
+
+        if (MACHINE.breakScheduled) {
+            MACHINE.breakScheduled = false;
+            return fail(baselib.exceptions.makeExnBreak("User break.",
+                                                        MACHINE.captureContinuationMarks(),
+                                                        // FIXME: capture the continuation as well,
+                                                        // rather than just hold false.
+                                                        false));
+        }
+
         var oldArgcount, oldVal, oldProc, oldErrorHandler, oldControlLength, oldEnvLength;
         if (! baselib.arity.isArityMatching(proc.racketArity, args.length - 4)) {
             var msg = baselib.format.format("arity mismatch: ~s expected ~s arguments, but received ~s",
@@ -351,4 +361,4 @@
     exports.rawApply = rawApply;
 
 
-}(this.plt.baselib, this.plt));
\ No newline at end of file
+}(this.plt.baselib, this.plt));
diff --git a/whalesong/js-assembler/runtime-src/runtime.js b/whalesong/js-assembler/runtime-src/runtime.js
index e82a612..0670583 100644
--- a/whalesong/js-assembler/runtime-src/runtime.js
+++ b/whalesong/js-assembler/runtime-src/runtime.js
@@ -106,6 +106,7 @@
 
     // Exceptions and error handling.
     var raise = baselib.exceptions.raise;
+    var makeExnBreak = baselib.exceptions.makeExnBreak; 
     var raiseUnboundToplevelError = baselib.exceptions.raiseUnboundToplevelError;
     var raiseArgumentTypeError = baselib.exceptions.raiseArgumentTypeError;
     var raiseContextExpectedValuesError = baselib.exceptions.raiseContextExpectedValuesError;
@@ -313,6 +314,13 @@
         this.globals = {};
 	this.primitives = Primitives;
         this.exclusiveLock = new ExclusiveLock();
+        this.breakScheduled = false;
+    };
+
+
+    // Schedule a break the next time the trampoline begins.
+    Machine.prototype.scheduleBreak = function() {
+        this.breakScheduled = true;
     };
 
 
@@ -461,9 +469,29 @@
     //
     var recomputeMaxNumBouncesBeforeYield;
 
+
+    // Checks to see if we need to handle break.  If so, returns true.
+    // Otherwise, returns false.
+    var maybeHandleBreak = function(MACHINE) {
+        if (MACHINE.breakScheduled) {
+            MACHINE.breakScheduled = false;
+            MACHINE.running = false;
+            MACHINE.params.currentErrorHandler(
+                MACHINE,
+                makeExnBreak("User break.",
+                             MACHINE.captureContinuationMarks(),
+                             // FIXME: capture the continuation as well,
+                             // rather than just hold false.
+                             false));
+            return true;
+        }
+        return false;
+    };
+
     var scheduleTrampoline = function(MACHINE, f, release) {
         setTimeout(
 	    function() {
+                if (maybeHandleBreak(MACHINE)) { release(); return; }
                 MACHINE._trampoline(f, false, release);
             },
             0);
diff --git a/whalesong/repl-prototype/htdocs/repl.js b/whalesong/repl-prototype/htdocs/repl.js
index 6830609..e370f65 100644
--- a/whalesong/repl-prototype/htdocs/repl.js
+++ b/whalesong/repl-prototype/htdocs/repl.js
@@ -93,6 +93,7 @@ $(document).ready(function() {
 
     var interruptEvaluation = function() {
         console.log('interrupt evaluation');
+        M.scheduleBreak();
     };