A double-blind, randomized, crossover trial was undertaken to compare the effect of enoximone (150 mg, 3 times daily) and captopril (25 mg, 3 times daily) added to conventional therapy with diuretics in the treatment of 13 patients with severe chronic heart failure. Each treatment was continued for 1 month. Heart failure was due to idiopathic dilated cardiomyopathy in 6 patients and coronary artery disease in 7. Hemodynamic measurements were made at rest and during exercise, on entry to the study and after each treatment period. The cardiac index at rest was 1.9 ± 0.2 liters min-1 m2 (mean ± 1 standard deviation) and did not change with either drug. Systemic vascular resistance at rest decreased with enoximone (p < 0.05) and was unchanged with captopril. Systemic vascular resistance at peak exercise was not lowered by either drug. Both drugs caused an increase of cardiac index at peak exercise (p < 0.04) and a prolongation of exercise time (p < 0.05). No difference was detected between the hemodynamic response to the 2 drugs after 1 month treatment either at rest or during exercise.