This study tested the hypothesis that for similar degrees of left ventricular hypertrophy, subendocardial blood flow would be facilitated by the increased diastolic coronary perfusion pressure associated with arterial hypertension, as compared with hypertrophy produced by banding the ascending aorta. Left ventricular hypertrophy was produced with perinephritic hypertension in seven adult dogs and by banding the ascending aorta in nine adult dogs. Left ventricular/body weight ratios were 6.15 ± 0.59 g/kg in the hypertensive animals and 6.87 ± 0.47 g/kg in dogs with aortic banding, as compared with 4.23 ± 0.23 g/kg in seven normal dogs (p < .01). Studies were performed at rest and during two stages of treadmill exercise to achieve heart rates of 195 and 260 beats/min. Diastolic aortic pressure was increased in animals with hypertension but not in dogs with aortic banding. Systolic ejection period was prolonged in dogs with aortic banding but not in hypertensive dogs. Mean blood flow per gram of myocardium measured with microspheres was similar at rest and during light exercise in all three groups of animals, whereas during heavy exercise blood flow was significantly greater than normal in both groups with hypertrophy. In normal dogs subendocardial/subepicardial (endo/epi) flow ratios did not change significantly during exercise. In both groups with hypertrophy, endo/epi ratios were normal at rest but decreased significantly during exercise. During heavy exercise the endo/epi ratio decreased to 0.73 ± 0.08 in dogs with aortic banding as compared with 1.07 ± 0.12 in hypertensive dogs (p < .01). These data suggest that the increased diastolic coronary perfusion pressure in hypertensive animals facilitated perfusion of the subendocardium of the hypertrophied left ventricle during heavy exercise.