Puoi fare cosi
codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Untitled</title>
<script language="JavaScript" type="text/javascript">
<!--
function controlla(aValue) {
if (aValue == '99') {
document.getElementById('t').value = 'tempo';
document.getElementById('t').style.display = 'block';
} else {
document.getElementById('t').value = '';
document.getElementById('t').style.display = 'none';
document.datiform.action='test3008.html';
document.datiform.submit();
}
}
//-->
</script>
</head>
<body>
<form name="datiform">
<select name="sel" onchange="controlla(this.value)">
<option value="pippo">Pippo</option>
<option value="pippi">Pippi</option>
<option value="pippa">Pippa</option>
<option value="pippu">Pippu</option>
<option value="pippe">Pippe</option>
<option value="99">99</option>
</select>
<input type="text" id="t" style="display:none">
</form>
</body>
</html>