Getting SelectedValue from RadioButtonList using javascript

[pierre]

Hi,

Can anyone tell me how to retreive the SelectedValue from a
RadioButtonList, using javavscript.

Here is my code:

<ASP:RADIOBUTTONLIST id="optOption" runat="server"
ONCLICK="javascript:alert(document.all.optOption.selectedItem);"></ASP:RADIOBUTTONLIST>

This is not working. I am getting an error saying that
'document.all.optOption.selectedItem is null or not an object'

Thanks in advance,
Pierre