Reakce na odpověď

Takto by to mělo fungovat:

<html>
<head>
<script type="text/javascript">
<!--
function zobraz(hodnota)
{
  var theOptionValue = hodnota.options[hodnota.selectedIndex].value
  alert(theOptionValue);
  return true;
}
//-->
</script>
</head>

<body>

<form name="muj_form" method="post">
  Vyberte mesic:
  <select name="form_vyber_mesic" id="menu" onchange='zobraz(this.form.form_vyber_mesic);'> 
    <option value="a">1</option>
    <option value="b">2</option>
    <option value="c">3</option>
    <option value="4">4</option>
    <option value="5">5</option>
    <option value="6">6</option>
    <option value="7">7</option>
    <option value="8">8</option>
    <option value="9">9</option>
    <option value="10">10</option>
    <option value="11">11</option>
    <option value="12">12</option>
  </select>
</form>           

</body>
</html>

Zkráceně:

theOptionValue = this.form.form_vyber_mesic.options[selectedIndex].value

JavaScript: ziskani hodnoty formulare - options