<select onchange="if(this.value=='hidden')document.all.mytext2.style.display='none'; else document.all.mytext2.style.display='block'">
<option value=1>1
<option value=2>2
<option value=hidden>hidden
</select>
<input id=mytext><br>
<input id=mytext2>
<option value=1>1
<option value=2>2
<option value=hidden>hidden
</select>
<input id=mytext><br>
<input id=mytext2>
<option > 11111</option>
<option > 22222</option>
</select>
<input type=text id="id1" value=1111>
<input type=text id="id2" value=2222>
<script>
function aa(obj){
if(obj.selectedIndex==0)
document.all.id1.style.display = "none";
else
document.all.id2.style.display = "none";}
</script>