The effects of the angiotensin converting enzyme (ACE) blocker temocapril on the capillary network of the left ventricle were studied in stroke-prone spontaneously hypertensive rats (SHRSP). The ACE blocker was dissolved in the drinking water and supplied to 24 and 32 week old SHRSP ad libitum for 5 weeks. The capillaries of the wall of the left ventricle were studied using a double staining method to differentiate the arteriolar, intermediate and venular capillary portions. Capillary density increased and capillary domain areas decreased in all capillary portions compared with untreated control SHRSP in both age groups. The proportion of venular capillary portions was increased by temocapril treatment. The results indicate that the late start of ACE blockade caused the regression of the hypertrophied cardiomyocytes, which is characteristic of SHRSP, and the rearrangement of capillary portions. The plasma concentration of angiotensin II was significantly lower in temocapril-treated SHRSP compared to the control group. The implication is that intrinsic angiotensin II exerts an appreciable effect on the function, structure and capillary network in the left ventricular wall in SHRSP.