Hypophysectomized, immature female rats, approximately 65 g in body weight, were implanted subcutaneously with a single anterior pituitary from 3- to 4-monthold female rats of the same strain, and were not treated further or were injected with thyroxine for 30 days. The effects on body and tail growth, mammary development and weight of several organs were measured. Hypophysectomized rats given a single pituitary transplant only, or a pituitary transplant together with thyroxine, exhibited body weight gains equal to 46.6 and 75.5 %, respectively, of that of intact controls. Tail length, tibial cartilage width, and spleen and thymus weights were all significantly greater in the rats with a pituitary transplant; addition of thyroxine resulted in a further increase in body weight, tail length and in spleen and thymus weight. No effects of the pituitary transplant or thyroxine were observed on the atrophic ovaries, uterus or adrenals. In contrast to the atrophic mammary glands of the hypophysectomized control rats, rats with a pituitary transplant showed significant mammary duct and alveolar growth near the site of implantation. It is concluded that in immature hypophysectomized rats: (a) an anterior pituitary transplant can synthesize and release significant amounts of growth hormone and prolactin; (b) thyroxine injections can induce enhanced body growth gains in the presence of the pituitary transplant, probably by increasing growth hormone release from the transplant, by synergizing with the released growth hormone, or by both actions; and (c) mammary development is elicited by a direct action on the mammary gland of growth hormone and prolactin released from the pituitary transplant, and this does not require the presence of ovarian or adrenal cortical hormones. (Endocrinology75: 565,1964)