The authors compared the response of myocardial blood flow to exercise in normal dogs and in dogs with left ventricular hypertrophy (LVH) produced by banding the ascending aorta at 6-9 wk of age. Blood flow was measured with 15-μm microspheres after the animals with LVH had reached adulthood when left ventricular:body weight ratios were approx. 80% greater than normal. During resting conditions, left ventricular systolic pressure was 202 ± 18 mm Hg in the dogs with LVH and 119 ± 6 mm Hg in the normal dogs (P<0.01). Three levels of treadmill exercise which increased heart rates to 190, 230 and 260 beats/min resulted in progressive increases in left ventricular systolic pressure to a maximum of 343 ± 18 mm Hg in the dogs with LVH as compared to 165 ± 10 mm Hg in the control dogs (P<0.01). Unlike normal dogs which showed a significant transmural perfusion gradient favoring the subendocardium at rest [mean subendocardial:subepicardial ratio (endo:epi) = 1.25 ± 0.07], subendocardial flow did not significantly exceed subepicardial flow in the animals with LVH (mean endo:epi = 1.10 ± 0.08; P>0.05 between normal and LVH). Myocardial blood flow increased as a direct linear function of heart rate during exercise in both groups of dogs. Exercise decreased the mean endo:epi ratio in both normal dogs (mean endo:epi = 1.10 ± 0.08 during heavy exercise; P<0.01) and in the animals with LVH (mean endo:epi = 0.94 ± 0.03; P<0.05), while the endo:epi ratios remained consistently less in the LVH dog than in the normal animals (P<0.05). The relative reduction of subendocardial flow in dogs with LVH was most apparent in the posterior papillary muscle region where the endo:epi ratio fell significantly below unity during heavy exercise (endo:epi= 0.79 ± 0.02; P<0.01). These data demonstrate that relative blood flow to the subendocardium of the left ventricle is significantly less than normal, both at rest and during exercise, in dogs with LVH produced by supravalvular aortic stenosis.