Russian physiologist Ivan Pavlov achieved scienific immortality for his discovery of the conditioned reflex. Pavlov's interests as physician and scientist were initially centered upon blood pressure, respiration and the digestive tract, which he studied in dogs. He received the 1904 Nobel Prize for this work. From about 1902 until his death, Pavlov concentrated on developing his theory of the conditioned reflex, demonstrating that psychological events are under physiological control at fundamental levels.