Se ho capito bene ecco l'esempio:
codice:
<script language="JavaScript">
    function cambia(val)
    {
    var valore=val.options[my_select.selectedIndex].value;

    document.getElementById("text").value=valore;
    }
</script>


<input type="text" id="text">


<select name="my_select" onchange="cambia(this)">
<option value="uno">uno</option>
<option value="due">due</option>
<option value="tre">tre</option>
</select>