Plasma testosterone level was measured in 52 rapists and 12 child molesters who had completed the Buss-Durkee Hostility Inventory, the Megargee Overcontrolled Hostility Scale, and the Michigan Alcoholism Screening Test. The rapists were classified according to the degree of violence during the commission of the rape. The ranges and means of the plasma testosterone level for the rapist and child molester controls were within normal limits. The group of rapists who were judged to be most violent had a significantly higher mean plasma testosterone level than normals, child molesters, and other rapists in this study. Mean Buss-Durkee hostility rating scores for rapists were significantly higher than the mean for normals, but there was no correlation between individual hostility scores and plasma testosterone levels. There was no correlation between age, race, or length of incarceration and plasma testosterone level.