The local antiandrogenic action of progesterone was investigated using the androgen-responsive flank organs of adult, sexually mature male Syrian golden hamsters. The effect of the unilateral injections of a micronized suspension of progesterone into the flank organs was analyzed by the measurement of the weight, area of surface pigmentation, and the cross-sectional area of the sebaceous glands. The weekly injections of 5 mg of progesterone for a period of 3 weeks produced approximately 50% reduction in all three parameters in comparison to the controls. The minimal effective dose of 1 mg per week was determined by the injection of progesterone at doses ranging from 0.1 to 5 mg. These effects were localized only to the treated flank organs since the values for the contralateral side were not significantly different from the controls. The local action of progesterone was further demonstrated by the absence of effect on the weight of seminal vesicles and testes of the treated animals in comparison to the controls.