Computations of compliance of the left ventricle (LV) during diastole assume passive tissue characteristics. To evaluate this assumption, we measured diastolic LV intramyocardial pressure simultaneously in the subepicardium and subendocardium in 18 open-chest dogs, using 1-mm in diameter micromanometers. Subepicardial pressure, 26 ± 1 mm Hg (mean ± SKM) exceeded subendo-cardial pressure, 14 ± 1 mm Hg (P< 0.001), and it exceeded left ventricular end-diastolic pressure (LVEDP) (9 ± 1 mm Hg) (P <0.001). After an infusion of dextran-40 (10 dogs), subepicardial diastolic pressure increased to 42 ± 4 mm Hg which was higher than diastolic subendocardial pressure, 26 ± 2 mm Hg (P< 0.001) and LVEDP, 24 ± 2 mm Hg (P< 0.001). Following cardiac arrest (12 dogs) with the intramyocardial probes unchanged in position, LV intracavitary pressure, 9 ± 1 mm Hg, and suben-docardial pressure, 13 ± 3 mm Hg, did not differ significantly from the pressures in the beating heart. Subepicardial pressure, 9 ± 1 mm Hg, was lower than in the beating heart (P< 0.001). Following distension of the arrested LV (12 dogs), subepicardial pressure, 31 ± 7 mm Hg, was lower than both subendocardial pressure, 58 ± 12 mm Hg (P< 0.001) and LV intracavitary pressure, 54 ± 11 mm Hg (P< 0.001). These observations indicate that tone is maintained by the subepicardium during diastole. Furthermore, the LV wall does not appear to behave as a passive shell during ventricular filling.Circ Res 47: 258-267, 1980