[Deff]

<form method="get" action="/cats/">
 <select name="cat_id" onChange="history(this);" >
 <option value="10" selected>10</option>
 <option value="25">25</option>
 </select>
 </form>
<script type="text/javascript">
function history(obj){
  window.location="/cats/?cat_id="+obj.value;
  }
</script>