Vespertilionid bats are thought to have a dissociated pattern of reproduction; mating occurs in autumn as well as during periodic arousals from hibernation when testes are regressed and ovaries are "in stasis." Sex steroid levels in both sexes are basal at this time. This pattern would indicate that sex steroids per se may not be activating sexual behavior. Gonadectomy of male and female big brown bats, Eptesicus fuscus, months before the mating season, did not significantly affect the probability of expressing sexual behavior in either sex when staged behavior tests were conducted. Ovariectomized females, in fact, mated significantly more and exhibited proceptive behavior. Males mated at the same frequency whether they were intact, orchidectomized, or given testosterone implants. These results argue that gonadal sex steroids are not the direct activators of sexual behavior in this species. However, when males were allowed to mate freely within a large flight cage, orchidectomized males were not seen to mate, while intact males mated at the same frequency as in the staged trials. Females are apparently either differentiating between the two groups of males or there is male-male competition for mates within a system that has multiple matings with different individuals in both sexes.