Ajax.NET

Dear sir:
I 'm writing a asp.net application.In this program,I writed aspx code like this:

<asp:dropdownlist id="testDemo" ..../>

then,I write a ajax.net method for set the dropdownlist's value.the client js code like this:
..
newOption.text=res.value.Tables[0].Rows[i].Description;
&#12288;&#12288; newOption.value=res.value.Tables[0].Rows[i].ProvinceID;
&#12288;&#12288; drp2.options.add(newOption);
....
so,I got the dropdownlist like I wanted.but when I want get the value of dropdownlist in aspx code,I lose! The aspx code just like this:(vb.net code)

..
ProvinceID = testDemo.SelectedValue
..
I can't get the testDemo's value,it seems that the javascript code can't affect the aspx code.

How can I do? the Ajax code can't integrate with asp.net easily???

please help me!