Previous studies using graded pulmonary artery (PA) occlusions were unable to determine whether the Frank-Starling relationship contributed significantly to right ventricle (RV) afterload tolerance. Hence, we wished to determine whether limitation of RV filling could decrease RV load tolerance. Anesthetized mongrel dogs were instrumented for the measurement of mean aortic pressure, pulmonary arterial flow, RV systolic (Ps) and diastolic (Pd) pressures, and myocardial segment length of the RV inflow (INF) and outflow (OUT) tracts. Filling of RV was limited by pericardial closure (PC). Before and after PC we performed graded occlusions of the PA to the point of cardiocirculatory failure (F). The magnitude of RV after load that could be tolerated without F was called the highest load tolerable (HLT). At HLT, with the pericardium open (PO), PA circumference had decreased by 33.9% ± 9.5%, and there were increases in Ps (from 28.3 ± 6.6 to 57.2 ± 17.7 torr, P < .001), Pd (from 4.4 ± 1.9 to 6.1 ± 1.5 torr, P < .01), and diastolic regional myocardial segment lengths (10.4 ± 4.2 to 11.6 ± 4.5 mm for INF, 10.4 ± 0.9 to 11.5 ± 0.5 mm for OUT; P < .05). Compared to PO, with PC, RV load tolerance decreased in that PA circumference could be decreased by only 26.2% ± 15.7% ( P < .05), and Ps increased to only 48.4 ± 22.5 torr ( P < .05). With PC, there was a significant decrease in end-diastolic fiber length for OUT, but not for INF. Further, with PC, at HLT, INF enddiastolic segment length increased (to 11.2 ± 3.3 mm) but, unlike with PO, there was no significant increase in OUT. However, for INF as well as for OUT, maximum diastolic length was significantly less with PC than with PO. By empirically fitting pressure-segment length curves to an exponential equation, we obtained an index of regional stiffness. Following PC, this was unchanged for the INF, but had increased significantly for the OUT. We conclude (1) the Frank-Starling relationship contributes to the capacity of the RV to tolerate increased afterload; and (2) the effects of pericardial closure on regional stiffness are nonhomogeneous, the outflow tract being more constrained than the inflow tract.