Right (RV) and left (LV) ventricular volumes, compliances, weights, and wall thicknesses were determined in the fresh post mortem hearts of 10 normal and eight pulmonary arterial banded (PAB) dogs. After two to 40 weeks of PA banding, the mean RV weight, 68·6 ± 5·4 g (mean ± SEM), was greater than normal (P < 0·001), with the maximum increase in RV weight occurring by the eighteenth week. Increases also occurred in the LV weights, but to a lesser extent and at a slower rate than in the right ventricle. Increases in wall thickness were greater at the ventricular bases than apices. In the PAB hearts, at 0 mm Hg (P0), the mean RV volume, 59·5 ± 3·8 ml./m2, and LV volume, 40·5 ± 2·5 ml./m2, were both greater than normal (P0 RV and LV volumes correlated negatively with duration of pulmonary arterial banding and correlated positively with increases in ventricular weight. In the PAB hearts, the mean RV compliance (ΔVav/ΔVLV) was decreased at P5 to P15 (P20. We postulate that an increase in RV wall tension resulting from PA banding causes an initial dilatation of the ventricular chamber and if sustained stimulates myocardial hypertrophy; subsequent increases in mass decrease the chamber volume and result in a decrease in wall tension, allowing the ventricle to reach a stable level of hyper-function.