Make sure the function is accessible to the page (i.e. declared public), and use the MasterType declaration in the ContentPage:
<%@ Page .... %>
<%@ MasterType VirtualPath="~/masterpage.master" %>
In the page, use Page.Master.MyFunction() to access the function.
*Note: before being able to access the function, you'll need to save & build.
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MCP - Web Based Client Development .NET 2.0