values in hidden control getting lost after postback

Hi,
I have two hidden controls:
<asp:HiddenField runat="server" id="pageHeader" />
<asp:HiddenField runat="server" id="pageInformation" />
I am calling following function from master page:
show_tip(this, document.getElementById('ctl00_pageInformation').value, document.getElementById('ctl00_pageHeader' Go to the complete details ...