codice:
<head>
<script> function MiaFunz(Valore){ 
var ref = document.getElementById('n0'); 
if (Valore=="Si"){ 
ref.style.display='block'; 
} 
else{ 
ref.style.display='none'; 
} 
} 
</script>
</head>

<body>

<select name="tetto" id="tetto" onChange="MiaFunz(gest.tetto.value);" >
               <option value="<%=rs("tetto")%>"><%=rs("tetto")%></option>
               <option value="Si">Si</option>
               <option value="No">No</option>
             </select>
<script> 
var sel = document.getElementById('tetto'); 
if (sel.options[sel.selectedIndex].value=="Si"){ 
document.getElementById('n0').style.display='block'; 
} 
</script>
<div style="display:none;" id="n0">
       <font face="tahoma">
         <small>
          <strong>Mq tetto:</strong> <input name="mq_tetto" type="text" id="mq_tetto" size="10" value="<%=rs("mq_tetto")%>">
         </small>
       </font>
</div>