The response to intravenous insulin was studied in seven diabetics after a dose of placebo, propranolol (40 mg), or metoprolol (50 mg). Two of the seven subjects also had a week's course of each of the same agents taken three times daily. Neither of the beta-blockers potentiated the effect of insulin as judged by the rate of reduction in blood-glucose. However, blood-glucose recovery was reduced significantly by propranolol, but not significantly by metoprolol. Propranolol caused severe bradycardia and raised diastolic blood-pressure during hypoglycaemia; these effects were milder with metoprolol. Propranolol inhibited the free-fatty-acid levels after hypoglycaemia to a greater extent than did metoprolol. The results strongly suggest that propranolol (and presumably other non-selective beta-blockers) is hazardous in subjects prone to hypoglycaemia. When diabetics require beta-blockade a cardioselective beta 1-blocker should be used.