codice:
<select name="select_1" onchange="if (this.value=='1') document.getElementById('select_2').display='block'; else document.getElementById('select_2').display='none';">
<option value="1">uno</option>
<option value="2">due</option>
<option value="3">tre</option>
</select>

<select name="select_2" id="select_2" style="display:none">
<option value="a">a</option>
<option value="b">b</option>
<option value="c">c</option>
</select>