The objective of this double-blind, placebo-controlled, randomized multicentre study was to determine whether treatment with the new non-sulfhydryl angiotensin converting enzyme (ACE) inhibitor quinapril, as an addition to maintenance therapy with digitalis and/or diuretics, would improve exercise tolerance and patients' symptomatology over a treatment period of 3 months. Two hundred and twenty-five patients with mild to moderate heart failure were studied in four parallel treatment groups receiving either placebo or quinapril in a dose of 10, 20, or 40 mg day-1. Compared to placebo, quinapril therapy resulted in a significant improvement in exercise time. The improvement in exercise tolerance was dose-related and showed a significant increase at the end of the study in the patients who completed the trial and in an intent-to-treat analysis. Twenty-six patients were on monotherapy with quinapril without concomitant medication. In this subgroup of patients, the increase in exercise time was comparable to the subgroup of patients on diuretic treatment alone. After the 12-week study, 189 patients entered an open-label trial for 12 months, in which the dose of the ACE inhibitor could be adjusted according to clinical response. Despite a reduction in the daily dose of quinapril, the patients reached the same level of exercise capacity as in the 3-month study. No serious side effects were recorded, particularly no symptomatic hypotension or deterioration of renal function. The results of the study show that quinapril has a significant favourable effect on exercise tolerance and symptoms in patients with mild to moderate heart failure.