The correct point in time to read an ASP .NET 1.1 control property in order to change page output

Question

Using ASP .NET 1.1 with CMS 2002 and C#. Trying to read the contents of a CMS HTML placeholder control into a string reader object so certain logic can be performed and the page output modified before it is displayed:

StringReader srHtml = new StringReader(this.HtmlPlaceholderControl1.Html);      

The problem is that the Html property is empty during Page_Load, I've tried in a PreRender overload as well, but the property is still empty. The HtmlPlaceholder control does render html when the page load has completed. Which event should be handled in order to read the propery of the web control?