The concentration of dihydrotestosterone was measured by radioimmunoassay in nuclear and cytoplasmic extracts from rat ventral prostates. In the regenerating prostates of castrated rats treated with dihydrotestosterone for 4 days, the nuclear concentration of this steroid increased from approx. 70nM to 800nM as a linear function of the injected dose, whereas the cytoplasmic concentration remained relatively constant (70-130nM). Isotope-exchange measurements of nuclear androgen receptors by using [3H]methyltrienolone indicated that, although the concentration of nuclear dihydrotestosterone was several-fold higher than the concentration of androgen receptors, they were logarithmically related. The recruitment of prostatic cells into the growth fraction and the stimulation of 5 a-reductase activity were more directly correlated to the nuclear concentration of androgen receptors than to the total nuclear concentration of dihydrotestosterone. Maximal restoration of a specific isoenzyme of acid phosphatase was achieved when approx. 2000 androgen receptors were present in the prostatic nuclei; higher concentrations of nuclear androgen receptors were associated with decreased amounts of this enzyme. Hence the results imply, first, that the total amount of dihydrotestosterone accumulated by nuclei is not a direct consequence of carrier-mediated transport by androgen receptors, and, secondly, that, whereas acid phosphatase may be differentially controlled by androgens in the regenerating prostate, increases in the amount of cell proliferation and 5a-reductase activity directly parallel increases in the nuclear concentration of androgen receptors.In the prostate, androgenic regulation of physiological functions and cellular growth is attributed largely to the formation, and subsequent binding to chromatin, of androgen-receptor complexes (King & Mainwaring, 1974 (Coffey, 1974;Rennie et al., 1975;Van Doorn et al., 1976). Although it has been postulated that androgen receptors are involved in the uptake and retention of dihydrotestosterone by target-cell * To whom correspondence should be addressed.Vol. 200 nuclei (Rennie & Bruchovsky, 1973;Bruchovsky et al., 1975a), a direct stoicheiometric relationship has not been demonstrated. When castrated rats are injected with radioactive androgen in the ventral prostate the concentration of radioisotope measured in nuclei exceeds that measured in the cytoplasm (Bruchovsky et al., 1975b). However, the concentration of androgen-receptor complexes recovered from the nuclei is insufficient to account for the nuclear influx of androgens if a mole-to-mole relationship is assumed. Furthermore, radioimmunoassay measurements of dihydrotestosterone in ventral prostates from non-castrated rats have indicated that only about 20% of the endogenous nuclear androgen is bound to androgen receptors (Bruchovsky et al., 1980). Whether the large nuclear concentration of non-receptor-bound dihydrotestosterone is achieved through a receptor mechanism or