The enzyme steroid 5 alpha-reductase (5 alpha-reductase) catalyzes the reduction of delta 4,5 double bonds in a variety of substrates and is thought to play both catabolic and anabolic roles in steroid hormone metabolism. Here, we describe the isolation and characterization of a cDNA encoding the rat type 2 isozyme of 5 alpha-reductase and compare the kinetic properties and tissue-specific expression patterns of this isozyme with those of the type 1 isozyme. The type 2 isozyme has apparent Km values in the nanomolar range for steroid substrates, whereas the type 1 isozyme has micromolar affinities. The isozymes differ in their inhibition by various 4-azasteroids with the type 2 isozyme showing exquisite sensitivity (Ki = 40 pM) to 21,21-pentamethylene-4-aza-5 alpha-pregn-1-ene-3,20-dione. Messenger RNAs encoding the type 2 isozyme are more abundant than type 1 mRNAs in most male reproductive tissues, whereas the type 1 mRNAs predominate in peripheral tissues. Both 5 alpha-reductase mRNAs are more efficiently induced by dihydrotestosterone than by testosterone in the regenerating prostate. The differences in substrate affinities and tissue distributions of the 5 alpha-reductase isozymes suggest that type 2 plays an anabolic role and type 1 a catabolic role in the metabolism of androgens and other steroid hormones.