Before working on conditioned reflexes, Pavlov completed a research program designed to investigate the physiological controls of salivary, gastric, and pancreatic secretions. The originality and significance of this work resulted in a Nobel Prize in 1904, the first awarded to a physiologist. The first part of this manuscript focuses on the experimental analysis of the controls of gastric secretion. This demonstrates Pavlov's experimental skill and his ability to draw strong inferences about how a psychic factor like appetite grew into a physiological effect, such as gastric secretion. The second part of the manuscript reviews Pavlov's ideas about the central neural mechanism for appetite and its stimulatory effect on gastric secretion, and demonstrates that Pavlov's "conceptual nervous system" is very similar to our own.