The effects of androstanediol and estradiol on prostatic growth were investigated in castrate dogs. Estrogens along resulted in no significant change in prostatic weight, whereas androstanediol produced growth comparable to that in uncastrated controls. Androstanediol plus estradiol resulted in an even more striking increase in prostate growth. Approximately half the animals receiving androstanediol alone and all of those receiving androstanediol plus estradiol fulfill the weight and histological criteria for prostatic hypertrophy in the dog. Since both these steroid hormones are presumed to be normal secretory products of the testis, it is possible that they are involved in the pathogenesis of prostatic hypertrophy in the dog.