This study reports the effects of the antilibidinal drug, medroxyprogesterone acetate (MPA) on nocturnal penile tumescence (NPT) during sleep, tumescence responses to provocative audiotapes during wakefulness in the laboratory, and sexual behaviors in the hospital environment, in a conduct-disordered sexually aggressive male, who had not responded to previous treatment. NPT, laboratory tumescence, and sexual behaviors in the hospital environment were all decreased. Testosterone levels declined by 73% within 2 weeks of starting the drug and prolactin levels increased by 300% across the treatment period. The effects on NPT and laboratory arousal were still partially in evidence 2 months after the drug had been stopped, although testosterone levels had returned to predrug levels. Some differences between the NPT and daytime test data are discussed in terms of the probable importance of testosterone to erotic fantasy. NPT may be a useful addition to present methods for examining the effects of drugs used to suppress socially unacceptable sexual behaviors.