asp.net radiobuttonlist selectedindex in javascript

Hello

I need to get the selectedindex of a asp.net radiobuttonlist in a javascript, anyone know how to do this?

The radiobuttonlist is declared like this:
------------------------------------------------------------------
<asp:radiobuttonlist id="rbl_menu_atgard" runat="server" AutoPostBack="true" OnSelectedIndexChanged="rbl_menu_atgard_OnChange">
<asp:ListItem Value="Scenario">Scenario</asp:ListItem>
<asp:ListItem Value="Prognos">Prognos</asp:ListItem>
<asp:ListItem Value="Regleringar">Rapportera regleringar</asp:ListItem>
</asp:radiobuttonlist>

The generated html becomes like this:
-------------------------------------------------------------------
<table id="rbl_menu_atgard" border="0">
<tr>
<td><input id="rbl_menu_atgard_0" type="radio" name="rbl_menu_atgard" value="Scenario" onclick="__doPostBack('rbl_menu_atgard_0','')" language="javascript" /><label for="rbl_menu_atgard_0">Scenario</label></td>
</tr>
<tr>
<td><input id="rbl_menu_atgard_1" type="radio" name="rbl_menu_atgard" value="Prognos" onclick="__doPostBack('rbl_menu_atgard_1','')" language="javascript" /><label for="rbl_menu_atgard_1">Prognos</label>
</td>
</tr>
<tr>
<td><input id="rbl_menu_atgard_2" type="radio" name="rbl_menu_atgard" value="Regleringar" checked="checked" onclick="__doPostBack('rbl_menu_atgard_2','')" language="javascript" /><label for="rbl_menu_atgard_2">Rapportera regleringar</label>
</td>
</tr>
</table>



when i retrieve the radiobuttonlist in a javascript like this:
var menu = document.getElementById('rbl_menu_atgard');

i can't access the selectedindex and options array. I need help on how to solve this.

please

/Nick