The response of sheep to intravenous insulin administration in doses of 0.5-10.0 units per kg. body weight has been studied in detail. The rate of fall of blood glucose following insulin injection is considerably slower than in non-ruminants. Increasing the dosage of insulin from 1 to 5 units per kg., and sometimes to 10 units per kg., does not increase the depth of hypoglycaemia, but merely increases its duration. The blood-glucose level following such doses of insulin falls, in most sheep, to a level of 5-10 mg. per cent. and remains relatively constant until it returns to normal. At these levels, severe hypoglycaemic signs were never observed, even when the hypoglycaemia was of several hours' duration. These results are compared with those obtained by others on cattle and goats and it is suggested that the response of sheep to insulin is similar in all respects to that shown by cattle and goats. From a consideration of the experimental results, it is suggested that the differences in response to insulin between ruminants and non-ruminants may be due to fundamental differences in endocrine balance which, in turn, are conditioned by differences in intermediary metabolism.