To determine if prolactin secreted endogenously by anterior pituitary grafts could augment male accessory organ weights, single anterior pituitary grafts were placed under the kidney capsule of male rats, whereas control animals received a graft of muscle. Three weeks after transplantation, the animals were sacrificed by decapitation and a significant increase in plasma prolactin was observed, which was accompanied by a highly significant increase in the weights of seminal vesicles, ventral and dorsal prostates, and adrenals. To determine the importance of testicular steroids in the response, animals were castrated. The increases in prolactin, seminal vesicle and ventral and dorsal prostate weights still occurred whether or not a small dose of testosterone replacement therapy was employed. In the presence of the pituitary graft, adrenal weight usually increased in these animals as well. To rule out a requirement for adrenal steroids in the response, adrenalectomized-castrate animals were also studied, and the increases in prolactin and prostate weights still occurred although there was no significant increase in the size of the seminal vesicles. To rule out the participation of other pituitary hormones, pituitary grafts were also placed in hypophysectomized animals, and in these animals, there was also a significant increase in prolactin, accessory sex organ and testicular weights, but the adrenals did not increase in size. The grafts failed to alter gonadotropin titers in any experiment. It is concluded that single anterior pituitary grafts are capable of secreting sufficient prolactin to increase the size of the sex accessories and sometimes the testes and adrenals of the rat. Furthermore, the effects on the male sex accessory organs were demonstrable in the absence of testicular or both testicular and adrenal steroids. The data suggest that prolactin may play a physiological role in the growth of the testes, adrenals and sex accessories.