Forty-three male Sprague Dawley rats, 6 to 9 months old, were given a single injection of 20 mg of diethylstilbestrol dipropionate and killed 406 days later. Compared with a control group of uninjected rats of similar age maintained under identical conditions, 30 of the injected rats had enlarged pituitary glands of which 24 were in a range from 12.8 to 16.8 mg and 6 were over 30 mg (37 to 318 mg). The 24 rats with slightly enlarged glands showed no marked abnormalities in pituitary and serum prolactin and growth hormone concentrations. In the 6 grossly enlarged pituitary masses, prolactin concentrations were very low and growth hormone concentrations were also low. In 5 of these rats, serum prolactin was elevated and serum growth hormone was elevated in one. It is concluded that the grossly enlarged pituitary masses were tumors induced by the single dose of estrogen and that the moderately enlarged glands represented a state of unresolved hyperplasia.