Testosterone and dihydrotestosterone are believed to exert their androgenic effects by interacting with a single intracellular receptor protein in androgen target tissues. During fetal life, however, testosterone mediates the virilization of the Wolffian ducts into the epididymis, vas deferens, and seminal vesicles, whereas the urogenital sinus and external genitalia require the in situ conversion of testosterone to dihydrotestosterone to undergo male development. The reason why the signal provided by testosterone needs to be amplified in some androgen target tissues but not in others remains an enigma. To provide insight into the different actions of these androgens we studied their interaction with the human androgen receptor in fibroblasts cultured from the genital skin of a patient with 5 alpha-reductase deficiency. Dihydrotestosterone was formed in negligible amounts in these cells, and in some experiments the residual 5 alpha-reductase activity was further blocked with the 5 alpha-reductase inhibitor finasteride. Saturation analysis in fibroblast monolayers disclosed similar amounts of binding with testosterone and dihydrotestosterone, and the affinity of binding of dihydrotestosterone was, on the average, about 2-fold greater than that of testosterone. [3H]Testosterone also exhibited a 5-fold faster dissociation rate from the receptor than [3H]dihydrotestosterone. In thermolability experiments the [3H]testosterone-receptor complex displayed marked instability at 42 C with 2 nM [3H] testosterone, whereas with 20 nM [3H]testosterone, receptor stability was similar to that seen with [3H]dihydrotestosterone. In up-regulation experiments, 2 nM [3H]testosterone produced a 34% increase in specific androgen receptor binding after 24 h, whereas 20 nM [3H]testosterone produced an average increase of 64%. Our results suggest that the weaker androgenic potency of testosterone compared to that of dihydrotestosterone resides in its weaker interaction with the androgen receptor, most clearly demonstrable as an increase in the dissociation rate of testosterone from the receptor. When present in relatively high concentrations, however, testosterone overcomes this defect by mass action.(ABSTRACT TRUNCATED AT 400 WORDS)