<script language="javascript">
function ocultaMuestra() {
if(document.forms[0].gato[0].checked)
document.forms[0].campo.style.visibility = "visible"
if(document.forms[0].gato[1].checked)
document.forms[0].campo.style.visibility = "hidden"
}
</script>
<form>
Muestra<input type="radio" name="gato" onClick="ocultaMuestra()">
<br>
Oculta<input type="radio" name="gato" onClick="ocultaMuestra()">
<br>
<input name="campo" style="visibility:hidden" value="ahora me veo!!">
</form>