Hypertrophic pyloric stenosis was observed to occur naturally in laboratory rabbits. Three cases in juvenile and adult rabbits and their antemortem histories were examined. The gross and microscopic pathology were studied. It was shown that not only the pylorus, but the stomach as a whole was increased in size both relatively and absolutely. The thickness of the pylorus was doubled, but its circumference was not increased, in hypertrophic stenosis. The chief histologic finding was hypertrophy of circular and longitudinal smooth muscles and their enveloping fascia. The myenteric plexi were essentially normal in appearance. The literature on a similar condition in man, dog, and cat was reviewed.