<html>
    <head>
    </head>
<script>
function combo(thelist, theinput) {
     theinput = document.getElementById(theinput);
     var idx = thelist.selectedIndex;
     var content = thelist.options[idx].innerHTML;
     theinput.value = content;
 }
</script>    
<body>
<input type="text" id="theinput" name="theinput" />
<select name="thelist" onChange="combo(this, 'theinput')">
   <option>one</option>
   <option>two</option>
   <option>three</option>
</select>
    </body>
</html>