The steroid hormone androstenedione profoundly influences the development and expression of sexual and aggressive behavior. The neural basis of these effects are, however, poorly understood. In this study we evaluated androstenedione's ability to maintain vasopressin peptide levels in the gonadal steroid-responsive vasopressin cells of the bed nucleus of the stria terminalis and the centromedial amygdala, and their projections. Adult male rats were castrated and given testosterone, androstenedione or no hormonal treatment for five weeks. Their brains were then processed for vasopressin immunoreactivity. Androstenedione and testosterone treatment were equally effective in preventing the reduction of vasopressin immunoreactivity associated with castration. Androstenedione may therefore be able to mimic the effects of testosterone on testosterone-responsive neural systems.