Hi Fran -
It's about half ads and about half articles and has been that way for many, many years (I think we're at 46 years and counting.)
A lot of our subscribers and advertisers know each other - when I was a subscriber and not the editor, I wanted to see the ads as much as the articles, in most cases.
So for most (obviously not all) of our subscribers it seems to be a balance that they're pretty happy with. It's also the balance that has let us keep the rates flat (even down a little for international) in a bad economy.
FWIW.
Lisa