<html xmlns="http://www.
<head>
...
<script language="javascript">
function show(){
obj=document.getElementById("city");
alert(obj.value);
}
</script>
...
<body>
<select name="city" id="city" onchange="show()">
<option value="111">北京市</option>
<option value="112">天津市</option>
<option value="113">上海市</option>
</select>