In this study, we report the synthesis and biological evaluation of four 6- and 17-substituted progesterone derivatives (7-10). These compounds were prepared from the commercially available 17alpha-acetoxyprogesterone. The biological effect of these steroids was demonstrated in in vivo as well as in vitro experiments. In the in vivo experiments, we measured the activity of 6-10 on the weight of the prostate glands of gonadectomized hamsters treated with testosterone (T). For the studies in vitro, we determined the IC(50) value by measuring the concentration of steroidal derivative that inhibited 50% of the activity of 5alpha-reductase present in the human prostate. The results from this work indicated that compounds 6-9 significantly decreased the weight of the prostate as compared to testosterone-treated animals and this reduction of prostate weight was comparable to that produced by finasteride. Steroid 8 was the most effective of the tested compounds. However, compound 10 did not exhibit this capacity. On the other hand, 6-9 exhibited a high inhibitory activity for the human 5alpha-reductase enzyme with IC(50) values of 10, 70, 22, and 19 nM, respectively. However, 10 was not effective for the inhibition of 5alpha-reductase activity. In conclusion, the compounds that contained the acetate ester moiety in the molecule (6, 7, 8, and 9) inhibited the activity of 5alpha-reductase and decreased the weight of the prostate. Nevertheless, the double bond in ring B seems to diminish the inhibitory potency (7 and 9), since 6, which does not possess a double bond at C-6, had the highest inhibitory activity (the lowest IC(50) value).