To the extent that they have deficient glucagon secretory responses to plasma glucose decrements, as they commonly do, patients with insulin-dependent mellitus (IDDM) are dependent on epinephrine-mediated β-adrenergic mechanisms to promote recovery from hypoglycemia. Thus, they are at increased risk for prolonged hypoglycemia if treated with a nonselective β-adrenergic antagonist such as propranolol. If the hyperglycemic actions of epinephrine are mediated through β2-adrenergic mechanisms, therapeutic efficacy (e.g., for hypertension or ischemic heart disease) could be accomplished without increased risk of hypoglycemia by selective β1-adrenergic blockade in such patients. However, oral administration of the relatively selective 9β1-adrenergic antagonist metoprolol (100 mg) and of the nonselective β-adrenergic antagonist propranolol (80 mg) both impaired recovery from insulin-induced hypoglycemia in patients with IDDM. Thus, at a dose of 100 mg, oral metoprolol is not safer than oral propranolol with respect to recovery from hypoglycemia in patients with IDDM.