Hi Mike Ullerup,
The problem when you are using a MasterPage is that it is an INamingContainer and the ClientID of the RadSlidingPane is different from its ID. The DockedPaneId property of the RadSlidingZone actually takes the ClientID of a SlidingPane. That is why, in this case, you could set the DockedPaneId on the server:
sender, EventArgs e)
SlidingZone1.DockedPaneId = Pane1.ClientID;
On a side note, if you upgrade to Q3 2009 SP1 or later the markup you have used should work out of the box without the need of server code.
All the best,
the Telerik team
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