We compared pump function of left and right ventricles in isolated ejecting cat hearts in which the natural series arrangement between the two sides of the heart was broken. The relationship between mean ventricular pressure and output obtained by varying the arterial load, the so-called pump function graph, and the mean external power output found at the various load levels, was taken as the basis for the comparison. The way in which pressures and flow generated by the right ventricle are changed by alterations in resistance and compliance of the loading arterial system, is qualitatively similar to what has been found in a previous study for the left side of the heart, in spite of the structural differences. The right ventricular pump function curve differs from the left ventricular curve in the absolute scale for the mean pressure axis, but if the scale is adjusted appropriately, the two curves look alike. Both ventricles appear matched to the input impedances of their respective arterial systems because both of them generate the maximum external power under the normal loading conditions. Right ventricular pump function is dependent on the left ventricular contraction mode. The presence of an isovolumic beat on the left side of the heart enhances right ventricular pump function. This is in contrast to the very small effect of the right ventricular contraction pattern on the pump function of the left side of the heart. Circ Res 46: 564-574, 1980