Treatment of mice with prolactin was previously demonstrated to increase the weight of the seminal vesicles in the presence and in the absence of testosterone and to promote accumulation of esterified cholesterol in the testes. To test whether these effects are part of the physiological role of prolactin in the male, we have examined the effects of inhibitors of prolactin release, ergocornine and 2-bromo-α-ergokryptine (CB-154), on the weight of the seminal vesicles and on the concentration of esterified and free cholesterol in the testes of mice. Ergocornine was administered as subcutaneous implants and CB-154 was suspended in oil and injected daily. To check their effectiveness, ergocornine implants were administered also to female mice. Implantation of a single ergocornine capsule in female mice prevented conception for a period of 2–3 weeks but did not seem to interfere with the estrous cycle. In lactating females, ergocornine implants caused a loss of litters or considerable impairment of their growth. In male mice, implants of ergocornine or injections of CB-154 caused a significant decrease in the weight of the seminal vesicles, and in the concentration of esterified cholesterol in the testes. The concentration of free cholesterol in the testes, the weight of the testes, and fertility were not affected. It is concluded that prolactin is normally involved in the regulation of the growth of accessory reproductive glands and the accumulation of esterified cholesterol in the testes of mice.