te he echo un programita haber si te vale:
Código HTML:
<html>
<head>
<script type="text/javascript">
function Aukeratu(a)
{
var b=a.selectedIndex;
var c=a[b].value;
alert(c);
}
</script>
</head>
<body>
<select name="bat" id="bat" onChange="Aukeratu(this)">
<option value="cero">cero</option>
<option value="uno">bat</option>
<option value="dos">bi</option>
</select>
</body>
</html>