Ethanol at an average blood concentration of 1 mg. per milliliter enhanced the immediate (first-phase) and prolonged (second-phase) insulin response to an intravenous glucose load in nonfasting normal human subjects. Simultaneously, the glucose disposal rate was increased and the postglucose hypoglycemia was accentuated, resulting in definite hypoglycemic symptoms in some individuals. Oral glucose tolerance was not changed by ethanol administration, but the thirty-minute blood glucose and plasma insulin values were increased, suggesting that alcohol might accelerate the absorption of glucose from the gut. Ethanol given orally during evening hours (1.5 gm. per kilogram) caused a nocturnal hyperinsulinemia and a decrease of blood glucose, but not an actual hypoglycemia. Oral glucose tolerance and plasma insulin response tested the next morning, when ethanol had disappeared from the blood, were not influenced by drinking the previous evening. The K-value of intravenous glucose was increased at this time, however. When alcohol was administered for one week at a dose corresponding to 25 per cent of daily calories and substituting for fat, both the oral and intravenous glucose tolerances were impaired in each subject but the insulin response remained unchaged. In obese nondiabetic subjects, ethanol did not potentiate the early insulin response to intravenous glucose but it increased the second phase of insulin secretion in response to sustained hyperglycemia. In contrast to conditions in nonobese subjects, the glucose disposal rate was not incresed and postglucose hypoglycemia was not accentuated by ethanol in overweight subjects. In insulin-deficient diabetic patients the absent early insulin response could not be restored by ethanol, and the late component of insulin release was little increased by alcohol infusion. Ethanol did not improve the glucose utilization of diabetic patients.