Abstract Investigations of cytosol receptor binding of androgen in brain have been carried out using either labelled testosterone or labelled dihydrotestosterone as ligand. The affinities of the binding of the ligands are in the same order of magnitude, competition shows that unlabelled excess of one can prevent binding of the other and both the testosterone and dihydrotestosterone high affinity binding moieties are rather uniformly distributed in brain. However, it is possible that these observations might conceal the existence of selective binding of one or other of the two ligands. Cytosols were prepared from brain and ventral prostate in adult male rats 2 days and 3 days after orchidectomy and from brain in intact adult females. In each cytosol the capacities and affinities of saturable, high affinity binding of both [ 3H]-dihydrotestosterone were measured by Scatchard plot analysis. Twenty-one pairs of observations were made with brains from females and although the extreme differences in ligand binding capacities between pairs was about 5-fold, there was near perfect correspondence in estimates of the saturation capacities for the two ligands. Similarly, while the abundance of binding sites fell between the 2nd and 3rd day after orchidectomy (by 50% in brain and 80% in ventral prostate) the ratios of the saturation capacities were not significantly different from 1.0.