The paper presents evidence indicating that, in an “in vitro” system, progesterone may interfere with the conversion of testosterone into its “active” metabolites (5 alpha-androstan-17 beta-ol-3-one, 5 alpha-androstan-3 beta, 17 beta-diol and 5 alpha-androstan-3 alpha, 17 beta-diol, etc.) both at the anterior pituitary and prostatic levels. Progesterone probably acts as a preferential substrate for the enzyme 5 alpha-reductase present in these structures. Other steroids are also active in diminishing the 5 alpha-reduction of testosterone in its target organs. Both corticoids and progestagens seem to be effective inhibitors for such a reduction. These observations open a new approach for developing anti-androgenic compounds.