<form name="form1">
<select name="sel"></select>
</form><script language="javascript">
var opt = new Option("YourText","YourValue");
document.form1.sel.options.add(opt);
</script>
<select name="sel"></select>
</form><script language="javascript">
var opt = new Option("YourText","YourValue");
document.form1.sel.options.add(opt);
</script>
<select name=s onchange="alert(this.value)">
<option value="aa">aa</option>
</selct>
<input type=button onclick="a()" value=add>
</form>
<script language=javascript>
var i=0
function a()
{
b=document.f.s
var opt=new Option("text"+ i,"value"+i,true,true)
b.options[b.options.length] = opt
i++
}
</script>