((HtmlTitle) Page.Master.FindControl("lblTitle")).Text =
    "Set some kind of title";

In most of the cases you need to change the master page’s title, you do not have to bother about this. Just use the correct page stadium and set the page’s title. The only page event to be able to do this is Page_PreInit (since master page and content page are still separate ‘pages’ in this stadium). The following will work (if you have AutoEventWireup enabled ):

protected void Page_PreInit(object sender, EventArgs e)
{
    Title = "Set some kind of title";
}