The ACE inhibitors cilazapril and captopril were administered at 10 and 100 mg/day, respectively, to spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto rats (WKY) from the 12th to the 22nd week of life. Both drugs produced statistically significant reductions in systolic and diastolic blood pressure, left-ventricular mass and index of left-ventricular hypertrophy in SHR. After cilazapril treatment, the morphology of SHR cardiocytes became similar to that in untreated normotensive rats, while in captopril-treated rats, myofibrils were disarranged, obliquely oriented and smaller than normal, with areas of electron-transparent sarcoplasm separating the myofibril bundles; mitochondria were also altered. In WKY rats, we observed no statistically significant changes in blood pressure, ventricular weight and hypertrophy index between the two drugs; however, there were different effects of the two drugs on the ultrastructural morphology of the myocardium. These observations suggest that these two molecularly dissimilar ACE inhibitors act differently at the tissue level despite similar effects on blood pressure and left-ventricular mass.