Testosterone alone stimulated spermatogenesis in four adult, pituitary stalk-sectioned rhesus monkeys. Ten to 14 weeks after transection of the pituitary stalk, testicular volumes declined to about one fifth of the presurgical values. Serum LH levels declined precipitously, being undetectable 1 week postsurgery, and serum testosterone levels were indistinguishable from those of castrated male monkeys by the 5th week postsurgery. After a transient decline, serum PRL levels increased to high values in all four monkeys throughout the rest of the postsurgery period. Twelve weekly injections of 250 mg testosterone enanthate resulted in peak testosterone levels around 25-fold higher than presurgical levels. Estradiol levels increased about 4-fold over presurgical levels, and PRL also increased further during the treatment phase. Small ejaculates were produced by electrostimulation by the 5th week of treatment. Thereafter, the ejaculate weight increased. Sperm were found from the 10th week in all four monkeys. By the 13th week of treatment, sperm counts in three monkeys ranged from 17-60 X 10(6) sperm/ejaculate. The sperm counts continued to increase for the first 4 weeks after the cessation of the testosterone enanthate injections. Thereafter, the sperm counts declined, and all four animals produced azoospermic ejaculates between 10 and 31 weeks posttreatment. Moreover, testosterone levels declined slowly and nonuniformly among the four animals. Testicular volumes declined and were at the lowest levels 14 weeks posttreatment. Estradiol and PRL levels also declined posttreatment. It is concluded that testosterone alone can stimulate spermatogenesis in stalk-sectioned rhesus monkeys even in the face of high serum PRL and estradiol levels.