codice:
<html> 
<head> 
<title></title> 
<script language="javascript"> 
<!-- 
function campo(opz,nome){

div = document.getElementById('divTesto') 
if(opz=='mar'||opz=='gio'||opz=='ven')
{ 
div.innerHTML='<select name="menu2"><option>non selez</option><option selected>selez</option></select>';
 alert('pianeta: '+nome)
} 
else 
{div.innerHTML='' }


} 
//--> 
</script> 

<body> 
<form name="myForm"> 
</form> 
<select name="menu1" onchange="campo(this.options[this.selectedIndex].value,this.options[this.selectedIndex].text)">
<option value='sel'>Seleziona un'opzione</option>
<option value='mar'>marte</option>
<option value='gio'>giove</option>
<option value='ven'>venere</option>
</select>
<div id="divTesto"></div> 
</body> 
</form>
ciao