The haemodynamic effects of the sulfhydryl-containing angiotensin converting enzyme inhibitor, zofenopril, were studied in patients in New York Heart Association functional class II and III. Twenty-one clinically stable patients with coronary artery disease or cardiomyopathy completed a randomized double-blind treatment period of 2 months with either 15 mg zofenopril once daily or placebo. Regular therapy with digoxin and diuretic drugs was continued. Left ventricular volumes were measured by radionuclide angiography at rest and during submaximal bicycle exercise.Zofenopril significantly increased mean stroke volume at rest from 59 to 67 ml (48 vs 48 ml in the control group, 95% confidence interval of the difference 1 to 16 ml) and left ventricular ejection fraction at rest from 39 to 43% (30 vs 30% in the control group, 95% confidence interval of the difference 1 to 8%). No significant changes occurred in heart rate, cardiac output, and blood pressure at rest, and zofenopril did not result in haemodynamic alterations during exercise.Thus, 15 mg of the sulfhydryl-containing angiotensin converting enzyme inhibitor, zofenopril, administered once daily to patients with moderate heart failure increases left ventricular function at rest, but not during exercise.