To detect the functional significance of subendocardial hypoperfusion in the pressureoverloaded left ventricle, we studied subendocardial and subepicardial function and subendocardial and subepicardial blood flow simultaneously in seven dogs with left ventricular hypertrophy (left ventricle/body weight ratio, 7.2 g/kg) produced by chronic aortic banding. Seven normal dogs served as controls. Subendocardial and subepicardial segment lengths were measured by ultrasonic dimension gauges, and myocardial blood flow was measured with radioactive microspheres. Atrial pacing (180-200 beats/min for 5 minutes) was used to produce a chronotropic stress. In dogs with left ventricular hypertrophy, the subendocardial blood flow failed to increase during pacing compared with the baseline state (1.21 ± 0.17 vs. 1.22 ± 0.17 ml/min/g). Subendocardial shortening fraction deteriorated with pacing stress (before pacing, 30.6 ± 3.9%; after pacing, 24.2 ± 3.7%; <0.001). In controls, subendocardial blood flow increased from 1.32 ± 0.19 to 1.80 ± 0.19 ml/min/g during pacing, and shortening fraction was preserved (before pacing, 25.5 ± 3.9%; after pacing, 25.9 ± 3.3%). Subepicardial blood flow in dogs with hypertrophy increased from 1.54 ± 0.24 to 2.32 ± 0.34 ml/min/g, and subepicardial shortening fraction was maintained (before pacing, 10.4 ± 1.0%; after pacing, 10.5 ± 1.2%) as it was in controls (subepicardial blood flow, from 1.27 ± 0.18 to 2.12 ± 0.17 ml/min/g; shortening fraction, from 16.6 ± 2.5% to 15.5 ± 2.2%). We conclude that, with pacing stress in pressureoverload hypertrophy, subendocardial blood flow failed to increase. This abnormality corresponded with a deterioration in subendocardial contractile function.