The present study assessed the effects of supraphysiologic doses of testosterone on aggressive responding in a controlled laboratory setting. Eight male subjects received gradually increasing doses of testosterone cypionate (150 mg/week for two weeks, 300 mg/week for two weeks, and 600 mg/week for two weeks) or placebo using a double-blind, randomized, cross-over design. Subjects were tested both before and after the series of injections. During the experimental session subjects could press a button to accumulate points exchangeable for money (non-aggressive response) or press another button to subtract points from a fictitious opponent (aggressive response). Aggressive responding was instigated by subtracting points from the subject which was attributable to the fictitious opponent. Testosterone administration resulted in a significantly higher number of aggressive responding compared to placebo.