Rat testes and accessory sex organs were perfused in situ by recirculating an artificial medium through the hemicorpus preparation previously developed for studies of skeletal muscle. The advantages and limitations of this system for studying the male productive tract were examined. The electrolyte and gas composition of the perfusate remained constant and glucose levels did not fall below normal during 3 h of perfusion. Testicular water content, temperature, and ATP and GTP levels were normal at 90 min. The mean arterial pressure was 40 mm Hg and the flow rates, measured with microspheres, were normal to high to the caput epididymides, ventral prostate and seminal vesicles and approximately half normal to the testes in preparations from 90 day old rats perfused at 35 ml/min. Administration of vasodilators indicated the absence of significant vasoconstriction in the hemicorpus. There was appreciable testosterone metabolism by the preparation and in addition, there was absorption of testosterone by the plastic tubing of the perfusion apparatus. Testosterone levels in the perfusate rose for 90 min in response to hCG. There was a dose-response relationship between hCG (20-1000 mIU/ml of medium) and testosterone levels at 90 min. FSH, prolactin, insulin and vitamins had no significant effect on hCG-stimulated testosterone levels. This perfusion system should prove useful for studies of hormone action.