Cardiac output studies were performed in 6 patients with complete heart block during four kinds of ventricular rhythm. These include ventricular or His-bundle pacing at the control rate, right ventricular outflow tract catheter electrode pacing, and both synchronous and atrial synchronous pacing by means of a Cordis Synchrocor unit. The latter two rhythms are defined as those with P wave, ventricular stimulus and QRS complex sequences, and atrial stimulus, P wave, ventricular stimulus and QRS complex sequences, respectively. Thirty-four paired observations were made during equal atrial and ventricular pacing rates in these patients. The importance of atrial systole for maintenance of cardiac output is clearly demonstrated by these data.