The effects of postextrasystolic potentiation during spontaneous ventricular ectopy on both left and right ventricular function were studied in 12 patients with the aid of list-mode radionuclide ventriculography. The left ventricular ejection fraction showed a significant increase (+ 11.0 +/- 5.0%; p less than .001) with associated mild increases in end-diastolic volumes (+ 4.9 +/- 4.6; p less than .01) and significant decreases in end-systolic volumes (-13.4% +/- 7.3%; p less than .001). A more heterogeneous response was seen for the right ventricle. Right ventricular ejection increased significantly in 8/12 patients (+ 3.3 +/- 3.0%; p less than .02). Despite a large increase in end-diastolic volumes (+ 10 +/- 7.9%; p less than .001), there were only mild changes in end-systolic volumes (+ 2.2 +/- 9.0; p = NS). It is concluded that, for spontaneous ventricular ectopy in man, the increase in right ventricular ejection fraction reflects improved ventricular filling whereas the increase in left ventricular ejection fraction is linked to improved emptying.