The pharmacological activity of eight pregnane derivatives 17-α acetoxyprogesterone 9, 17-α acetoxy-4, 5-epoxypregnan-3, 20-dione 10, 17-α acetoxy-4-chloro-4-pregnene-3, 20-dione 11, 17-α acetoxy-4-bromo-4-pregnene-3, 20-dione 12, 17-α hydroxy-4-bromo-4-pregnene-3, 20-dione 13, 4-chloro-17-α hydroxy-4-pregnene-3, 20-dione 14, 17-α benzoyloxy-4-bromo-4-pregnene-3, 20-dione 15 and 17-α benzoyloxy-4-chloro-4-pregnene-3, 20-dione 16 was aeter, mined. These compounds were evaluated as antiandrogens on gonadectomized hamster seminal vesicles.The pharmacological data in this study indicate that compounds 15 and 16 having a C-17 benzoyloxy moiety showed the highest antiandrogenic activity as measured by the reduction of the weight of the seminal vesicles, followed by the steroids 11 and 12 (17-α acetoxy group). The free alcohols 13 and 14 exhibited a lower antiandrogenic activity. Apparently, the ester moiety at C-17 is a necessary requirement for the presence of high antiandrogenic activity. shows the inhibitory effect on the conversion of testosterone (T) to DHT, of the above described steroids as measured by the amount of produced DHT 2 expressed as pmoles of DHT/g of protein/h.Steroids 11, 12 and 16 showed a much higher inhibitory activity on the conversion of testosterone (T) to dihydrotestosterone (DHT) than presently used finasteride 3.