Cardiac output was estimated immediately before and after conversion to sinus rhythm in nine patients with rapid arrhythmias. Conversion was by synchronized direct-current shock in eight patients, and by direct atrial wall stimulation in the other. In seven patients there was an immediate increase in cardiac output after restoration of sinus rhythm. The percentage increase in output was directly proportional to the rate of the arrhythmia immediately before conversion (r=0.91, P<0.01). The critical heart rate, above which an immediate increase in cardiac output might be expected on conversion to sinus rhythm, appeared in these patients to be about 160 beats per minute.