Rehola.
Código HTML:
<html>
<head>
<script>
function cambiar(val) {
if(val==1)
document.getElementById('txt1').disabled=true;
else
document.getElementById('txt2').disabled=true;
}
</script>
</head>
<body>
<select onchange="cambiar(this.value)">
<option value="1">1</option>
<option value="2">2</option>
</select>
<input type="text" id="txt1" />
<input type="text" id="txt2" />
</body>
</html> Saludos,
