The hormonal and cardiovascular responses to intravenous (i.v.) insulin were studied in 16 insulin-dependent normotensive diabetic patients after acute injection of selective beta-1-adrenoceptor blocking agents. The lowest blood glucose levels were not affected by beta-1-adrenoceptor blockade while the time for reaching nadir was significantly reduced. Plasma adrenaline levels increased significantly during selective beta-1-adrenoceptor blockade. Plasma noradrenaline, glucagon and human growth hormone levels, however, remained unaffected. The insulin-induced tachycardia was not prevented by the beta-1-blockade, and the mean arterial blood pressure was unchanged. The ability to recognize the symptoms of insulin-induced hypoglycaemia persisted in all patients, although less pronounced in nine of them. We suggest that selective beta-1-adrenergic blocking agents may be employed without risk in the treatment of hypertensive insulin-dependent diabetic patients.