diff --git a/whalesong/repl-prototype/htdocs/index.html b/whalesong/repl-prototype/htdocs/index.html
index f7735b8..89259f2 100644
--- a/whalesong/repl-prototype/htdocs/index.html
+++ b/whalesong/repl-prototype/htdocs/index.html
@@ -10,6 +10,6 @@
 
 <div id="output"></div>
 <br/>
-<input id="repl" type="text" style="width:500px"></input>
+<input id="repl" type="text" style="width:500px"></input><img id="break" src="break.png"/>
 </body>
 </html>
diff --git a/whalesong/repl-prototype/htdocs/repl.js b/whalesong/repl-prototype/htdocs/repl.js
index 0101fd1..6830609 100644
--- a/whalesong/repl-prototype/htdocs/repl.js
+++ b/whalesong/repl-prototype/htdocs/repl.js
@@ -3,6 +3,10 @@ $(document).ready(function() {
 
     var repl = $("#repl");
     var output = $("#output");
+    var breakButton = $("#break");
+    breakButton.hide();
+    breakButton.click(function() { interruptEvaluation(); });
+
 
     var M = plt.runtime.currentMachine;
 
@@ -49,9 +53,11 @@ $(document).ready(function() {
                     $(this).val("");
                     repl.attr('disabled', 'true');
                     repl.val("... evaluating...");
+                    breakButton.show();
                     evaluate(src, 
                              function() { repl.removeAttr('disabled');
-                                          repl.val("");});
+                                          repl.val("");
+                                          breakButton.hide();});
                 } 
             });
         });
@@ -85,6 +91,10 @@ $(document).ready(function() {
 	}
     };
 
+    var interruptEvaluation = function() {
+        console.log('interrupt evaluation');
+    };
+
 
     // In evaluation, we'll send compilation requests to the server,
     // and get back bytecode that we should evaluate.