Pineal acid phosphatase (ACP) activity was examined in seven experiments involving young intact or castrated male and female Syrian hamsters. Nine-week old female hamsters had a 3-fold (p<0.001) higher ACP activity in their pineal glands than did males. After three weeks of castration, a significant increase (p<0.001) in ACP activity occurred in castrated male pineal glands. In males, pellets with 5α-dihydrotestosterone (p<0.01) but not testosterone or progesterone suppressed pineal ACP activity. In females, no changes in pineal ACP activity were noted due to the estrous cycle. Pineal ACP activity was not affected by testosterone, dihydrotestosterone or androstenedione pellets in intact females or by testosterone pellets in ovariectomized animals.