The capacity of the rat mammary gland to respond to testosterone stimulation with lobule-alveolar development only when growth hormone is present has, in these experiments, been used as a method for studying whether the pituitary gland, autotransplanted into the kidney capsule, can secrete growth hormone. Injections of 0.05 or 0.25 mg of testosterone propionate daily for 14 days did not stimulate any lobule-alveolar development in the mammary glands of castrated rats with autotransplanted hypophysis. When this treatment was given for about 4 weeks, a few alveoli were seen in the mammary glands. In castrated rats with intact pituitary gland the same doses of testosterone propionate stimulated an extense lobule-alveolar development even after only 14 days of treatment. Injections of testosterone propionate together with growth hormone in rats with autotransplanted hypophysis stimulated the same degree of lobule-alveolar development as did injections of testosterone alone in rats with intact pituitary gland. These observations on the mammary glands indicate that there is a considerable deficiency of growth hormone in rats with the pituitary gland autotransplanted into the kidney capsule.