The physiological role of gonadal androgens in regulating bone metabolism is not established. To determine if androgens antagonize the changes in cortical bone after gonadectomy, we treated orchiectomized (ORX) rats with testosterone (T) and 5 alpha-dihydrotestosterone (DHT), and ovariectomized (OVX) rats with the afore-mentioned androgens, as well as the synthetic androgen fluoxymesterone (Fl) and the nonsteroidal estrogen diethylstilbestrol (DES). OVX resulted in a rapid, sustained increase in periosteal bone formation at the tibial diaphysis, whereas ORX resulted in decreased bone formation. Androgen treatment stimulated bone formation in ORX rats and suppressed bone formation in OVX rats. A large dose of DES suppressed bone formation in OVX rats to values below the intact controls. The results of these studies demonstrate that androgens counteract the changes in cortical bone formation after gonadectomy in females as well as males, and thereby reestablish the sex difference observed in intact rats.