We planned this study to elucidate the combined effects of dobutamine and dopamine on right ventricular function in surgical cardiac patients at 12 to 24 hours after open-heart surgery. Firstly, the total dose of both drugs, which showed clinically to have the best effects in maintaining hemodynamic status, was determined and administered as an equal dose combination of dobutamine and dopamine (Phase 1). Subsequently the combination of dobutamine : dopamine was changed from that in Phase 1 (1 : 1) to : Phase 2 (1.5 : 0.5), Phase 3 (2. 0 : 0), Phase 4 (0.5 : 1.5), and Phase 5 (0 : 2.0), with the added dose of both drugs kept constant, and the hemodynamic status was compared between each phases. Right ventricular ejection fraction (RVEF) was measured by the thermal washout method with the aid of a Swan-Ganz catheter equipped with a rapid response thermistor, and right ventricular end-diastolic volume index (RVEDVI), right ventricular end-systolic volume index (RVESVI), right ventricular end-diastolic pressure (RVEDP), right ventricular compliance index (RV-C), and other hemodynamic variables were obtained. Compared to Phases 2 and 3 (in which dose ratios of dobutamine and dopamine were 1.5 : 0.5 and 2 : 0 respectively), Phases 4and 5 (at dose ratios of 0.5 : 1.5 and 0 : 2 respectively) showed significantly low levels of RVEF, stroke volume index (SVI), RV-C and heart rate (HR), the same level of RVEDVI, and significantly increased level of right and left ventricular filling pressures (RVEDP and PCWP), pulmonary vascular resistance index (PVRI), and systemic vascular resistance index (SVRI). Phase 3 (a double dose of dobutamine alone) increased the heart rate without improving any other hemodynamic parameters, when compared to Phases 1 and 2. Phases 2 and 1 (with dose ratios of dobutamine to dopamine being 1.5 : 0.5 and 1 : 1) showed the best positive inotropisms with the least chronotropic action and may there fore be the desirable choice for the support of hemodynamics in patients immediately after open-heart surgery. Moreover, Phases 4 and 5 (dopamine-dominantly administered groups) showed decreased right ventricular compliance in a reversed dose-related response to the administered dopamine, and such an effect on the right ventricular compliance may be one of the characteristics of dopamine.