A study of the respiration of rat ventral prostate has been carried out with special reference to the effect of added carbohydrate and certain sex steroids. The average Qo2 value for rat ventral prostate was 9·1. When this tissue was examined 48 hr after castration, there was a significant decrease in the average Qo2 value to 7·1. The rate of respiration remained the same in the absence and in the presence of added glucose or fructose. A high concentration of testosterone, 50 μg/ml., inhibited the oxygen consumption of ventral prostate; oestradiol had a less pronounced effect. With lower concentrations of testosterone, such as 0·5 μg/ml. or 0·005 μg/ml., variable results were obtained, but the average Qo2 value did not differ significantly from that established for the ventral prostate alone. The addition of testosterone to the ventral prostate from castrate rats did not increase the respiration.