To contrast the previously described distribution of 5 alpha-reductase isoenzyme 2 in human organs we have raised synthetic C-terminal peptides of human 5 alpha-reductase isoenzyme 1 (h 5 alpha r1) and isoenzyme 2 (h 5 alpha r2) respectively, and studied the cellular and subcellular distribution of both isoforms of this key enzyme of testosterone metabolism using the human prostate gland as a reference. h 5 alpha r1, which in Western blots of human prostatic proteins had an apparent molecular weight of 23 kDa, was localized immunohistochemically in the nucleus of prostatic epithelial and stromal cells. Ultrastructurally, it was closely associated with the nuclear matrix. The apparent molecular weight of h 5 alpha r2 was 26 kDa in Western blotting of human prostatic proteins. In immunohistochemically processed sections, it was seen in the cytoplasm of prostatic epithelial as well as stromal cells. An organ screening with genital and extragenital tissues of males and females was performed and to elucidate the distribution of the isoenzymes. Presence of both isoenzymes was demonstrated for a number of tissues, including the central nervous system, the urogenital tract, the respiratory tract, the gastrointestinal tract, skin, and the endocrine system. The divergent localization of 5 alpha-reductase isoenzymes points to potentially different functions in various organs. In view of the nuclear localization of isoenzyme 1, its close spatial relationship with the androgen receptor is presumed to indicate a close association with the receptor mechanism.