Left ventricular performance (parameters: EDV, ESV, EF, cardiac output) was studied by gated radionuclide ventriculography in 55 patients with ventricular or AV-sequential pacemakers. 23/55 patients were followed over 4-6 months. EDV and SV decreased significantly (p less than 0.01) and cardiac output increased (p less than 0.01) with changing pacing rate from 70-72 to 100/min. After 4-6 months a reduction of cardiac output was found in 15/23 patients. As compared to ventricular pacing, a significantly higher cardiac output was found during AV-sequential pacing. These nuclear data indicate the dependence of left ventricular performance on pacing mode. From non-invasive methods an individual adapted pacing mode can be derived.