EXPERIMENTS which have explored the effects of varying the amounts and ratios of estrogens and progesterone on mammary gland growth have been reported only for the rabbit (Lyons and McGinty, 1941; Scharf and Lyons, 1941) and for the mouse (Mixner and Turner, 1942a). It is surprising that these effects have not also been studied systematically on the mammary gland of the rat. The experiments reported here have attempted to assess the synergism and antagonism of estrogen and progesterone in the growth of the mammary gland of the immature ovariectomized rat. materials and methods Female rats of the Harvard colony were fed ad lib a diet of Rockland pellets, and were ovariectomized at twenty-four days of age. Subcutaneous injections of estradiol-17β, or of progesterone, were begun four days later. The daily dose of estrogen was always injected in 0.05 ml. sesame oil while progesterone was always given as a solution of 20.0 mg./ml. sesame oil.