In intact adult rats almost all androgen receptor (AR) sites of the rat ventral prostate (RVP) are occupied by endogenous dihydrotestosterone, and about 80% of these sites are nuclear. Nuclear AR disappears rapidly after castration (half-life of 3 h). The amount of cytosolic AR does not change within the initial 36 h, then markedly decreases during the next 2-5 days. An early and specific action of androgen is a remarkable increase of its own receptor. RVP also contains an estradiol receptor (ER) which rapidly disappears after castration and which, contrary to AR, is predominantly localized in the cytosol of stromal elements. The published procedures for steroid receptors grossly underestimate receptors concentrations in normal (NHP) and hyperplastic (BPH) human prostate. We have recently established a reliable method for the measurement of total AR, and we have found no difference in AR concentrations between NHP and BPH. BPH also contains a progesterone receptor and an elusive ER. Finally, we have used specific immunoglobulins in sex hormone binding plasma protein (SBP) for the demonstration of SBP-like immunoreactivity by the indirect immunofluorescence technique. The specific antigenic material was exclusively localized in the cytoplasm of BPH epithelial cells.