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We examined the effects of antihypertensive therapy with captopril, an angiotensin converting enzyme inhibitor, on exercise tolerance and humoral factors in 19 elderly patients (>60 years old) with essential hypertension. Captopril (37.5-75 mg/day) was administered for 8 weeks. Fourteen of the 19 patients in whom captopril was effective took a treadmill exercise test according to Kattus' protocol. Exercise tolerance was increased in all patients (from 13.1 ± 1.3 to 16.5 ± 1.0 min, P<0.01). Captopril attenuated the rise in blood pressure during the exercise test but did not affect the heart rate. Resting values of plasma adrenaline decreased by 47% and noradrenaline by 17%, with no significant changes in plasma renin activity (PRA) or aldosterone. The change in mean blood pressure showed an inverse relationship to pretreatment plasma noradrenaline (r=-0.73, P<0.01). The results show that captopril is effective in the treatment of hypertensive elderly patients, and suggest that the sympathetic nervous system is involved in the mechanism of the antihypertensive response to captopril therapy.