The 3-keto reduction of [1,2-3H]dihydrotestosterone1 to 3α- and 3β-androstanediol has been assessed in homogenates of 31 dog prostates that varied in size from 15 g. The formation of both androstanediols was demonstrable in microsomes and cytosol, and both NADH and NADPH were effective cofactors for the two reactions. The formation of 3β-androstanediol per unit weight of prostate or per mg protein did not show a clear-cut relation with the size of the gland, but 3α-androstanediol formation per mg microsomal protein increased as a linear function with size so that the rates were on an average 10-fold greater in prostates >15 g than in those 1The trivial names and abbreviations used are: dihydrotestosterone, 17β-hydroxy-5α-androstan-3-one; 3α-androstanediol, 5α-androstane-3α,17β-diol; 3β-androstanediol, 5α-androstane-3β,17β-diol.