The difference between the right (RV) and the left ventricle during progressive pressure loading is that the RV changes to cylindrical form without change in its enddiastolic pressure (EDP). We investigated this particular phase experimentally and clinically. In experimental animal studies (dogs), the gradual construction of the main pulmonary artery (PA) resulted in an elevation of RV systolic pressure (RVSP) without changes in RVEDP and PA flow (FPA). This was followed by an increase in RVEDP and a decrease in FPA. In this initial phase, the RV increased in its dimension from the free wall to the septum. In 155 clinical cases with RV pressure loading, the relationship between RVEDP and RVSP suggested that the RV compliance of the cases with chronic RV pressure loading was less than that with subacute loading. However, these studies did not supply the expected data on the phase we mentioned. In 2 patients with primary pulmonary hypertension who showed spontaneous remission during our observation, a decrease in RVSP without change in RVEDP occurred with a change in RV form detected by 2-dimensional echocardiography. These data suggested the existence of a phase in which the RV showed a character of plasticity during pressure loading, but direct documentation was not successful.