Così funziona...

codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Prova</title>

<script language="javascript" type="text/javascript">
<!--
function vediTesto(a){
	var e=document.getElementById(a);
	if (!e) return true;
	if(e.style.display=="none") {
		e.style.display="block";
	} else {
		e.style.display="none";
	}
	return true;
} 


//-->
</script>

</head>

<body>
<div onclick="vediTesto('myText');">mostra/nascondi</div>
<div id="myText">Testo</div>

</body>
</html>