Intendi cosi'?

codice:
<script type="text/JavaScript">
<!--
function MM_jumpMenu(targ,selObj,restore){ //v3.0
location = "pagina.php?variabile=" + selObj.options[selObj.selectedIndex].value;
if (restore) selObj.selectedIndex=0;
}
//-->
</script>

<form name='filtri'>
<select name='variabile' onchange="MM_jumpMenu('parent',this,0)">
<option value='1'>1</option>
<option value='2'>2</option>
<option value='3'>3</option>
</select>