Comparative sucrose gradient studies of the in vitro binding of dihydrotestosterone (DHT) and of a synthetic androgen, methyltrienolone (R 1881), have been done with the cytosols of various tissues of the rat, mouse, cock and man. With rat prostate cytosol, the amount of R 1881 and DHT binding in the 8–9S region of the gradient was found to be comparable. Specific 8-9S peaks of R 1881 were also found in rat levator ani/bulbocavernosus and skeletal muscles and in the mouse kidney. Only 4-5S peaks could be demonstrated in the cock’s comb while DHT under the same conditions showed both 8–9S and 4–5S binding. Binding of R 1881 to the cytosol of the hyperplasticprostate was polydispersed, and showed evidence of the presence of aggregates. Evidence was also found that R 1881 could bind to the progesterone receptor in rat uterus. Our study supports the theory that in a given species the androgen receptors are similar if not identical in all the tissues. The synthetic androgen R 1881 appears to be a useful tool for androgen receptor studies in various animal species provided that the tissue under study contains no progesterone receptor.