Few would disagree that reproductive behavior is a powerful aspect of human life and society. Of the many aspects of human sexuality (which include some elements of inter- and intrasexual aggression and competition), one of the most intriguing is the extent of biological influences. Intuitively, one feels that powerful emotional and behavioral conditions exhibited during sexual or aggressive arousal must have biological bases. What are the hormonal and physiological correlates of such profound behavioral displays? Employing a research strategy involving psychiatric and endocrine perspectives, this laboratory's approach is to challenge the nervous system, either directly with hormone and drug treatment, or indirectly through behavioral or environmental stimuli, and monitor biobehavioral changes in physiology, mood, and arousal. In particular, I am interested in how gonadal and pituitary hormones, chemical messengers important to the reproductive system, act to organize, mediate, and reinforce human sexual and aggressive behavior. At present, two main lines of inquiry have been opened to investigate different elements of human psychoendocrinology. First among these is an investigation into biological bases of human sexual differentiation and sexual orientation. The second involves studying the physiology of erotic arousal and competitive behavior.