The effects of low- and high-intensity endurance training on the capillary network of rat left ventricle were studied morphometrically. Eighteen male albino rats of Wistar strain (4-wk-old) were assigned at random to a sedentary control group (Cont, n=8) and two trained groups which were both subjected to exercise on a motor-driven treadmill for 60 min a day, 5 days/wk for 9 weeks from 7 wks to 16 wks of age with different running speed; the low-intensity trained group (T-20, n=5) ran at 20 m/min and the high-intensity trained group (T-40, n=5) at 40 m/min. All morphometric parameters for the capillary and muscle fiber were determined in perfusion-fixed hearts. After the training period, the average muscle fiber cross-sectional area in the T-20 and the T-40 was not significantly different from the Cont. There were no significant differences in the capillary density and the capillary-to-fiber ratio between any groups, suggesting no significant change in capillary number. On the other hand, the number of capillary with large luminal diameter (8-10 μm) in the T-40 but not the T-20 was significantly greater than the Cont. These results indicate that the high-intensity endurance training causes enlargement of the capillary luminal area, while neither the low-nor the high-intesnity endurance training stimulate the proliferation of capillaries in the left ventriclular myocardium. In conclusion, a structure of the capillary network of rat left ventricle responds to the high-intensity endurance training by enlarging capillary luminal area rather than by increasing capillary number.