1. The present study was designed to test the hypothesis that beneficial effects of angiotensin converting enzyme (ACE)inhibitors are independent of a fall in blood pressure in rat experimental heart failure following coronary ligation. 2. The animals were assigned randomly to six groups; sham operation, controls subjected to coronary ligation (control), coronary ligation plus chronic treatment with ACE inhibitors at non- and hypotensive doses; perindopril (0.2 or 2 mg kg-1 day-1) or enalapril (2 or 20 mg kg-1 day-1) for three weeks starting one week after the ligation. 3. Systemic blood pressure was measured every week during the experiments. At the end of the treatments, cardiac function and heart weight (an index of myocardial hypertrophy) were determined. In the other animals, ACE activities in plasma and tissues including heart, kidney, lung and blood vessels were measured. 4. In the controls, cardiac ACE activity, weight of right ventricle and left ventricular end-diastolic pressure (LVEDP) were higher compared to those in the sham-operated animals four weeks after the coronary ligation. However, ACE activities were not changed in plasma, kidney, lung and aorta by ligation of the coronary artery. 5. The chronic treatment with perindopril at a dose of 0.2 mg kg-1 day-1 inhibited the increase in ACE activity in cardiac tissue and suppressed the right ventricular hypertrophy without affecting systemic haemodynamics. In contrast, enalapril at a dose of 20 mg kg-1 day-1, but not 2 mg kg-1 day-1, prevented the development of the right ventricular hypertrophy. Enalapril at 20 mg kg-1 day-1 also lowered systemic blood pressure. 6. There is no significant correlation between systemic blood pressure and right ventricular hypertrophy at the end of the treatment with perindopril (r = 0.06) or enalapril (r = 0.1).7. These findings demonstrate that perindopril, an ACE inhibitor, prevents cardiac hypertrophy without affecting systemic blood pressure in the rat with heart failure after coronary ligation, and suggest that selective augmentation of ACE activity in cardiac tissue is involved in the progression of hypertrophy in this model.