<html>
<head>
<script type="text/javascript">
function label_option( el )
{
        var label = el.options[ el.selectedIndex ].text;
        alert( label );
}
window.onload = function()
{
        document.getElementById( 'teste' ).onchange = function()
        {
                label_option( this );
        }
}
</script>
</head>
<body>
        <form action="" method="post">
        <label>Teste: <select name="teste" id="teste">
                        <option value="">--</option>
                        <option value="1">Um</option>
                        <option value="2">Dois</option>
                        <option value="3">Tres</option>
                </select></label>
        </form>
</body>
</html>

referência:

http://www.mredkj.co…utorial002.html