This study was undertaken to assess ventricular volumes and function by radionuclide angiography during asynchronous ventricular pacing in a group of 22 patients who needed an artificial pacemaker. 14 had ischaemic heart disease and 8 had primary disturbances of conduction of the impulse-forming system. The transition from sinus to a paced rhythm or increasing the pacing rate had little effect on patients with primary conduction disturbances. In contrast, in patients with ischaemic heart disease, the transition to a paced rhythm decreased significantly left ventricular end-diastolic (15.3 ± 4.7%) and stroke (26.3 ± 4%) volumes, ejection fraction (9.0 ± 4.4%), and cardiac output (21.0 ± 3.9%). Pacing at a progressively increasing heart rate showed that each patient had an optimal rate of pacing.