Because of the differing opinions on the value of glucagon and the best way of administering it in severe hypoglycaemia induced by sulphonylurea drugs, experiments were conducted on normal subjects, and in diabetic patients before and during treatment with chlorpropamide. In all subjects glucagon raised blood sugar concentrations, and produced a greater and more prolonged increase in blood glucose concentrations when given intramuscularly than when given intravenously. Hypoglycaemia did not occur in any subject.