5 alpha-Androstan-3 alpha, 17 beta-diol (3 alpha-diol) is a well known metabolite of dihydrostestosterone (DHT) and the concentration of its glucuronide in serum is looked upon as a marker of DHT activity in skin and prostate. The data in this communication show that 3 alpha-diol is a potent agonist of the human and canine prostatic SHBG-[SHBG receptor]. It causes a robust rise in intracellular cAMP within minutes; the 1/2 max. response occurs at a concentration equal to its concentration in male plasma. Estradiol is the only other agonist in this system. All other steroids that bind to SHBG are antagonists.