The golden Syrian hamster flank organ has been used to study the stimulatory effect of androgens on sebaceous glands and hair. Androgens cause the sebaceous glands and hair follicles in this organ to grow. We have made the novel observation that exogenously administered androgen, testosterone propionate (TP), suppresses hair growth in the area surrounding the flank organ. When given in a time-release (systemic) subcutaneous dosage form (pellet), 25 mg TP inhibited the regrowth of clipped hair in peri-flank organ skin for up to 21 days; however, by 28 days hair grew back to the same extent as in controls. The peak serum level of testosterone in TP-treated animals occurred at 14 days, and declined thereafter. When two separate TP pellets (25 mg/pellet) were administered 14 days apart in order to maintain high serum levels for 28 days, the amount of hair regrowth after 35 days was identical to animals receiving a single TP pellet or placebo. This suggests that the systemic level of testosterone was not the only factor in hair regulation. Hair growing within the flank organ appeared to be unaffected by TP administration. In the golden Syrian hamster, androgen, as in humans, can exert stimulatory and inhibitory effects on hair growth depending on the body site. We conclude that this animal model could serve as a useful system to investigate the mechanisms responsible for the opposing effects of androgen on hair growth.