in the SelectedIndexChanged event of the DropDownList that is placed in UpdatePanel, I use the session object to stotre the selected value: Session["code"] = pathologiesDropDownList.SelectedValue; when the evnt is truggred the first time (not in the others) I got the error: Sys.WebForms.PageRequestParserErrorException: The message received from the server could not be parsed .Common cause for this error are when the response is modified by calls to Response.Write() , response filters , HttpModules , or server trace is enabled..... what's the problem?