The studies were designed to explore the effect of the converting enzyme inhibitor captopril on the activity of the sympathetic nervous system during basal conditions and following graded physical exercise in patients with essential hypertension. Seven males and two females, aged 36-59 years, were hospitalized under metabolic ward conditions and treated for 7 days with captopril given orally in increasing dosages, the final dose being 600 mg daily. The patients were subjected to an individual, graded submaximal work test (bicycling) for 20 min before medication and then again in an identical manner during medication with 600 mg captopril. Blood samples were drawn before exercise and then after 10 and 20 min of work for the determination of plasma angiotensin II (PA II), plasma aldosterone (PAC), plasma renin activity (PRA), plasma noradrenaline (PNA) and plasma adrenaline (PA). Before medication blood pressure (mmHg) was 195/133 immediately before exercise, 230/129 after 10 min of moderate exercise and 263/105 following 20 mon of nearly maximal work. During treatment with captopril the respective blood pressure values were 154/110, 200/100 and 245/98. Captopril had no significant effect on the changes in heart rate following physical exercise. PA II and PAC were substantially reduced and PRA considerably increased by captopril. PA II, PAC and PRA increased in response to exercise both before and following captopril. The exercise stimulated increase in PNA and PA was almost identical before and during captopril. Thus, captopril had no major effect on the activity of the sympathetic nervous system in patients with essential hypertension, neither during basic conditions nor during heavy physical exercise in spite of a profound decrease in PA II.