Abstract. The conversion of [1,2,6,7-3H]testosterone2 to [3H]dihydrotestosterone has been assessed in ground spongiosa of normal and osteoporotic bone. The tissue was obtained from 23 patients, 18 women, 3 men and 2 children who were undergoing orthopedic surgery. The formation of dihydrotestosterone was demonstrated in all samples examined. The half maximum rate of dihydrotestosterone formation occurred at a substrate concentration of 0.3 μm a value similar to that reported for dihydrotestosterone formation in other androgen target organs. Under the standardized conditions utilized in this study the rate of dihydrotestosterone formation did not differ significantly in normal as compared to osteoporotic bone. Furthermore, the conversion of testosterone to androstenedione was also similar in osteoporotic and normal bone. Based on these studies it seems reasonable to conclude that dihydrotestosterone rather than testosterone is the active intracellular androgen in human bone since in other androgen target tissues androgen action is mediated by dihydrotestosterone if 5α-reductase is present.