In the present study 12 patients with severe chronic heart failure (7 with dilated cardiomyopathy and 5 with ischaemic heart disease) underwent right heart catheterization at rest and during isometric exercise by handgrip before and 6 months after treatment with captopril (mean dose 2 X 39.6 mg/day) while concomitant digitalis and diuretics were unchanged. After 6 months resting haemodynamics changed as follows: decrease in heart rate, mean arterial pressure, pulmonary capillary pressure and total vascular resistance; increase in stroke volume index and stroke work index. During handgrip the following changes were observed after captopril: decrease in total vascular resistance; increase in stroke volume index and stroke work index. Before captopril 11 patients showed no increase or even a decrease in stroke work index during isometric exercise, whereas after captopril 5 out of the 12 patients showed an increase in stroke work index. The results demonstrate that in patients with severe chronic heart failure long-term treatment with captopril lowers preload and afterload, which is accompanied by a significant improvement in left ventricular performance at rest. Furthermore, during isometric exercise after captopril an improvement in left ventricular function was found in 5 out of 12 patients.