Estrogenic compounds are effective in stimulating live weight gains and improving feed efficiency of growing and finishing ruminants. With the proper use of diethylstilbestrol or estradiol, weight gain and feed efficiency are improved approximately 15 and 12 percent, respectively. The administration of estrogenic substances to finishing cattle or sheep results in heavier carcasses containing more protein and moisture and less fat. Estrogens do not appreciably affect rumen fermentation or digestibility of the diet but improve the utilization of the absorbed nutrients. Evidence to support the concept that the increased weight gained with estrogen treatment is the result of anabolic reactions includes a decrease in plasma urea, decrease in plasma levels of most of the essential amino-acids, decreased excretion of urea and total nitrogen in the urine and increased retention of nitrogen, phosphorus and calcium. There is no change in body water space or tubular reabsorption of urea in sheep treated with diethylstilbestrol to account for some of these observations. More direct evidence that estrogens exert their major influence on nitrogen metabolism in the body and more specifically on protein synthesis comes from an experiment where cattle fed diethylstilbestrol deposited 33 percent more protein and 18 percent less fat in body weight gain as compared with control animals. The use of the hormone resulted in a significant improvement in the efficiency of utilizing dietary protein for body gain and a slight decrease in the efficiency of converting dietary energy to body gain. Enlargement of the pituitary, thyroid and adrenal glands and higher plasma levels of growth hormone, insulin and glucose have been found in experiments designed to study the effect of estrogens on the endocrine system of ruminants. These studies, along with the observation that an intact pituitary gland is necessary for estrogens to increase nitrogen retention in sheep, support the theory that estrogens are anabolic in ruminants because of increased secretion of growth hormone by the anterior pituitary. If follows that the increased secretion of growth hormone results in increased blood glucose which then stimulates secretion of insulin. Both growth hormone and insulin would be stimulatory to protein synthesis. This theory of the mode of action is further supported by the findings that injections of growth hormone closely resemble the effects of diethylstilbestrol on nitrogen retention and blood metabolites in sheep.