To explore the possibility that the presence of propranolol-induced beta-adrenergic blockade might have an adverse effect upon homeostatic circulatory reflexes activated by the administration of a potent vasodilator agent, arterial blood pressure and pulse rate response to rapid intravenous diazoxide injection was monitored before and after pretreatment with propranolol in ten hypertensive patients. It appeared that beta-adrenergic blockade had no clinically significant effect on the magnitude of hypotension or the degree of heart rate acceleration induced by the administration of the potent vasodilator diazoxide. This reflex vasodilator-induced cardio-acceleration after propranolol adminstration could be the result of incomplete blockade of endogenously released neurotransmitter, inhibition of the parasympathetic nervous system, or a direct pharmacologic action of diazoxide. Diazoxide administration to hypertensive patients in the presence of beta-adrenergic blockade was not associated with any clinically significant hemodynamic consequences.