... così:

codice:
<select name="sel" onChange="this.form.campo.value=this[this.selectedIndex].value;">
<option value="v1">Valore 1</option>
<option value="v2">Valore 3</option>
<option value="v3">Valore 4</option>
</select>



<input type="text" name="campo">
Ciao !