Google is your friend, but keep in mind that "yoga" is an umbrella term for a large variety of exercises. In particular, yoga as an Indian discipline aimed at reaching moksha, the liberation from the reincarnation cycle, is rather different from yoga as practiced in the West with the goal of losing 10 lbs.

I saw one study that indicated that meditation did not lower blood pressure, refuting earlier studies, but that yoga did. Can't find it now however. The wikipedia page on meditation research might be useful. also this