$(document).ready(function()
{
    $('select#metro_state').change( function()
    {
        var metro_state = $(this).val();
        
        $.ajax({
            type: "POST",
            url: "/ajaxian.php",
            data: "action=get_metro_areas&metro_state=" + metro_state,
            success: function(msg){
                $('select#metro_city').html(msg);
            }
        });
    });
});