MORE THAN 60 years have elapsed since the discovery and isolation of the male sex steroid testosterone.1 Today we continue to strive to understand the physiologic and psychological mechanisms for male sex hormones. The studies by Pope et al,2 Rabkin et al,3 and Tuiten et al4 provide an important addition to our understanding of the effect of testosterone on the brain and on changes in mood, aggressive behavior, and sexual function. These studies advance our knowledge of testosterone as a psychoactive compound at supraphysiologic doses in healthy individuals and in those with human immunodeficiency virus (HIV) infection. The importance of this increased knowledge of the risks and benefits of testosterone is underscored by the widespread illicit use of anabolic compounds for strength, body building, and aesthetic purposes. The importance of these studies is also related to the high rates of sexual dysfunction in both men and women and impairment in gonadal function associated with HIV infection and other medical conditions.