1 The effect of captopril on autonomic reflex functions has been investigated in fifteen patients with essential hypertension by examining their responses to tests of baroreceptor function (Valsalva's manoeuvre and upright posture), sympathetic nervous system reactivity (cold pressor and mental stress tests) and parasympathetic reactivity (diving test) before and after 3 weeks' treatment with captopril. 2 Captopril significantly reduced arterial blood pressure and resting heart rate but did not affect cardiovascular responses to Valsalva's manoeuvre, upright posture, cold pressor and mental stress tests. 3 The bradycardia associated with the diving reflex test was significantly enhanced by captopril (P less than 0.01). 4 These results indicate that treatment with captopril is associated with increased parasympathetic tone but without inhibition of baroreceptor or sympathetic reflexes.