Pressure-overloaded hypertrophy of the left ventricle (LV) was produced by coarctation of the ascending aorta in 7 dogs. The overall mean weight of the left ventricle (LVW) was 7.86 +/- 1.49 (S.D.) g/kg body weight; (normal, 5.99 +/- 0.70 g/kg: p less than 0.05). After potassium arrest, pressure-volume (P-V) relationships were examined with the left ventricles isolated from the normals and from the dogs of left ventricular hypertrophy (LVH-dogs). In both groups, the P-V relationships could be expressed by an equation deltaV=a-be-cP throughout the range of filling pressure of 2.5 to 35 cmH2O, where deltav was the actual volume change of LV, P intraventricular pressure, and a, b and c constants. A sensitive index of LV stiffness, the half-inflation pressure (h), was defined as 1n (2b/a)/c. In hypertrophied hearts, h was 10.5 +/- 0.7 cmH2O; (normal 8.0 +/- 0.4 cmH2O; P less than 0.001). The ratio of LVW to LVVp=h (the left ventricular volume at h) in hypertrophy, which was related to the LV chamber geometry, was 3.1 +/- 0.6 in contrast with the normal value of 2.0 +/- 0.3. The development of concentric hypertrophy was thus demonstrated. Moreover, h was closely correlated with LVW/LVVp=h in both the normals and the LVH-dogs (r=0.83; p less than 0.01). On the other hand, an index of LV wall stiffness h/LVW/LVVP=h was relatively constant. Therefore, the increase of LV stiffness in the LVH-dogs was attributed to the change in chamber geometry.