The objective of this study is to synthesize new steroidal compounds based on the progesterone skeleton with a high inhibitory activity for the enzyme 5α-reductase. Presently similar compounds are being used for the treatment of androgen dependent diseases such as: hirsutism, androgenic alopecia, bening prostatic hyperplasia and prostate cancer. Dihydrotestosterone 2 (Fig. (1)), a 55α-reduced metabolite of testosterone 1 has been implicated as a causative factor in the progression of these diseases, largely through the clinical evaluation of males who are genetically deficient of steroid 55α-reductase enzyme. As a result of this study, the inhibition of this enzyme has become a pharmacological strategy for the design and synthesis of new antiandrogenic drugs. The advent of finasteride 8 (Fig. (4)) a 55α-reductase inhibitor has grately alleviated the symptoms associated with benign prostatic hyperplasia. In our laboratory we recently synthesized several new 165β-methyl-pregnadiene-3,20-diones derivatives 27 (Fig.(6)), 38-42 (Fig. (11)), 16β-phenyl-pregnadiene-3,17a-dione derivatives 32-33 (Fig. (7)), 16β-phenylpregnatriene- 3,17a-diones, 30, 31 (Fig. (7)) and 16β-methyl-pregnatriene-3,20-diones 43-46 (Fig. (11)). These compounds were evaluated as 5α-reductase inhibitors in the following biological models: Penicillium crustosum broths, the flank organs of gonadectomized male hamsters, the incorporation of radiolabeled sodium acetate into lipids, the effect of the new steroids on the reduction of the weight of the seminal vesicles and on the in vitro metabolism of [3H]T to [3H]DHT in seminal vesicles homogenates of gonadectomized male hamsters. All trienones 30, 31, and 43-46 in all biological models showed consistently a higher 5α-reductase inhibitory activity than the corresponding dienones 27, 32, 33 and 38-42. We believe that with these compounds the 5α-reductase enzyme is inactivated by an irreversible Michael type addition of the nucleophilic portion of the enzyme to the conjugated double bond of the steroid. The trienones having a more coplanar structure react faster with the enzyme and thus show a higher inhibitory activity.