The volume and pressure of one ventricle have been demonstrated to modulate the volume and pressure in the contralateral chamber during systole and diastole. To quantitate the isolated systolic effects of left ventricular (LV) pressure on right ventricular (RV) mechanics, we rapidly withdrew blood from the LV immediately after diastole via an apex cannula during a single cardiac cycle in eight open-chest, open-pericardium anesthetized pigs (45 kg) and studied the effects on the RV. Reductions in LV pressure of up to 75 mm Hg were achieved in midsystole without changing LV or RV diastolic volume or pressure. Resultant changes in RV flow and pressure development during these single unloaded beats may therefore be considered to result from pure systolic interaction. The instantaneous left-to-right systolic pressure gain [G(t)] was determined as the ratio of RV pressure change to LV pressure change as a function of time during systole, and the mean LV-to-RV systolic pressure gain was determined as the ratio of changes in mean systolic RV pressure to changes in mean systolic LV pressure. During LV unloading, there was an average reduction of 62.6 ± 12.3%in the mean systolic LV pressure, which resulted in decreases of 13.6 ± 6.4% in mean RV systolic pressure, 17.9 ± 10.4% in RV stroke volume, and 27.0 ± 113% in RV stroke work. G(t) was found to vary significantly within systole, reaching a minimum of 0.042 ± 0.014 mm Hg/mm Hg at normalized time 0.70 of the systolic duration and a maximum of 0.079 ± 0.029 at the end of RV ejection. The midsystolic value for G(t) was 0.055 ± 0.028, and the mean systolic gain was 0.054 ± 0.017 mm Hg/mm Hg. These results demonstrate that, independent of diastolic conditions, there is a substantial transmission of systolic forces from the LV that contributes to RV ejection and that the pressure interaction gain varies within the systolic portion of the cardiac cycle. (Circulation Research1992;70:944–951)