Toggle Display (On / Off)

<script language="javascript" type="text/javascript">
  function TDisplay(elementID) {
  var CurrentElement = document.getElementById(elementID);
  CurrentElement.style.display = (CurrentElement.style.display != 'none') ? 'none' : '';
  return false;
}
</script>

<asp:LinkButton ID="LinkButton1" runat="server" OnClientClick="return TDisplay('panel1');">
Toggle Display(On / Off)</asp:LinkButton>
<asp:Panel ID="panel1" runat="server">
    Panel content here!
</asp:Panel>