Spontaneously hypertensive rats (SHR) were studied to test the hypothesis that endurance exercise training can stimulate capillary growth and offset the decrement associated with the development of myocardial hypertrophy. The exercise group (SHR-T) was trained on a treadmill for 10 weeks at 70-90% maximum VO2 and compared to nontrained SHR and normotensive Wistar-Kyoto (WKY) at 16 weeks of age. Thus, the training program coincided with the development of hypertension and hypertrophy in SHR. Image analysis was used to study capillaries in one micron thick left ventricular tissue samples from perfuse-fixed hearts. Training did not affect left ventricular mass or blood pressure, but reversed the characteristic decrements in capillary surface area (CSA), volume (CV), and numerical density (CD). CSA and CV were most markedly affected by exercise, as mean values for these parameters increased by 31 and 40%, respectively, compared to SHR. The magnitude of these changes approximated the magnitude of hypertrophy as evidenced by left ventricular weight/body weight ratios (42% in SHR and 37% in SHR-T). Anatomical intercapillary distance was also normalized by training (means +/- SEM): SHR-T, 11.65 +/- 0.31; SHR, 13.97 +/- 0.37; WKY, 11.19 +/- 0.37. These data indicate that exercise stimulates capillary growth in the face of developing hypertension and its related left ventricular hypertrophy.