Because changes in adolescent boys are correlated with a major increase in the production of testosterone, many researchers have concluded that testosterone is the primary biological factor responsible for normal and perhaps abnormal sexual behavior. Further support for this conclusion is provided by reports that hypogonadal men experience a return of libido and potency when administered exogeneous testosterone. While there is obviously some link between adequate testosterone levels and sexual arousability, other factors such as social expectations may also be important. Studies of testosterone-level responses to erotic stimulation, the treatment of hypogonadal men with testosterone, behavioral effects of castration, and the link between testosterone and aggression present conflicting results. Even those studies reporting an association among general aggression, sexual aggression, and androgen levels have either not been replicable or the differences observed have been of marginal clinical significance. When evidence of disturbances in early life and family characteristics are well-established in individuals who have committed sexual assault, the results of hormone studies do not appear very impressive. While there is the possibility of some subtle abnormality in androgen receptors or neurotransmitters, additional research is needed. 32 references, 1 figure.