The feeding of 2.5 mgm. of diethylstilbestrol (DES) per head per day to growing wethers fed a dehydrated grass meal ration increased dry matter intake by 6 per cent and decreased dry matter digestibility by 2.4 per cent. The DES-treated lambs digested 5.4 cent (P<.05) more crude protein, 9.2 per cent (P<.20) more ether extract, 15 per cent (P< .01) less crude fibre, and 3.5 per cent (P<.01) less nitrogen-free extract. The use of a two stage in vitro digestion technique did not reveal any influence of a wide range of DES levels upon dry matter digestibility. The implantation of 12 mgm. DES in growing ewes and wethers increased pituitary and adrenal gland weights by 46 per cent (P<.005) and by 23 per cent respectively. The thyroid gland weights were decreased 21 per cent (P<.005). The gross histology of the pituitary gland was not found to be changed by DES treatment The use of I¹³¹ did not reveal changes in the activity of the thyroid gland following the feeding of 1.0, 2.0, and 2.5 mgm. DES per day to growing wethers. Lambs fed an all-concentrate ration and implanted with 12 mgm. DES gained more rapidly and efficiently than their controls The DES-treated lambs had a lower dressing percentage, and required less time to reach market weight. These results indicate that the action of DES in enhancing growth rate and efficiency of gain in the ruminant appears to be mediated by one or more mechanisms. First, DES may alter the digestive activities of the rumen microflora by selectively promoting their proteolytic activity and inhibiting their cellulolytic activity. It may, nevertheless, allow for an increased absorption of the degradation products of protein metabolism. These products may be present in a more readily digested form or may be more readily passed through the intestinal tract wall. Secondly, DES may stimulate growth of the anterior pituitary, resulting in an altered production of a number of the tropic hormones arising from this gland. With an increased growth of the anterior pituitary, an increased level of STH could account for a more efficient feed utilization. Control and DES-treated lambs utilize approximately the same amount of apparent digestible energy, however DES-treatment resulted in an increased growth rate. This must be due to an increased utilization of net energy, possibly associated with an alteration in tropic hormone production. / Land and Food Systems, Faculty of / Graduate