Increased sympathetic activity occurs in congestive heart failure and may have deleterious effects. To examine the effects of angiotensin converting enzyme (ACE) inhibition on sympathetic activity in heart failure, the noradrenaline response to exercise was measured in 18 patients given enalapril or placebo in double-blind trial. The plasma noradrenaline response to graded exercise was significantly reduced after 4 weeks on active treatment but was unchanged by placebo treatment. The rate-pressure product at maximal exercise was significantly reduced after 4 weeks in the enalapril group but unchanged in the placebo group. These results suggest that ACE inhibition reduces sympathetic activity in patients with heart failure.