In order to set the text in the RadTooltip you must first get a reference to it via the $find() method and then use its client-side API and more precisely the set_text(newText) method.
All the best,
Marin
the Telerik team
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As long as the ClientID you pass to the function is correct and both pages come from the same domain so you can access them via JavaScript things will work. You must also make sure your code is executed at all, for example add an alert. I am attaching here a simple page that works: http://screencast.com/t/cilp55KQssd. You can use this as base for further developing this functionality, for example call a function from the main page that will take the new text as a parameter and get the reference by itself.
Regards,
Marin
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed now.