ho fatto così ma non funziona!

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//IT">
<html><head>

<script type="text/javascript">
function testo() {
if (!document.forms['form1'].elements['vari'].checked) {
document.form1.docUscita.value='';
}
else
{
document.form1.docUscita.value='miao';
}
}
</script>
</head>

<body leftmargin="0" topmargin="0" marginwidth="0" marginheight="0">

<div align="center">

<form name="form1">
<input name="docUscita" maxlength="20" size="20">
<input name="vari" value="true" type="checkbox" onchange="testo();"> Docs vari
</form>

</div></body></html>