A new method for quantitative assessment of the atrial contribution to ventricular ejection in sequentially paced patients is described. The atrial contribution (AC) has been defined as the pulse pressure decrement (invasive arterial measurement by a canulla inserted into the brachial artery), expressed in percent of the control pulse pressure, induced by switching off the atrium activating impulse for one beat In 17 patients, the AC was found to be atrioventricular (AV) interval dependent, the measurements were well reproducible (the mean difference between two measurements at different times was 93 %, S.D. 8.4). For the AV interval of 170 ms, it was found to be 29.3 % (+8.9) in patients with the sick sinus syndrome, 27.0 % (+3.2) in patients with complete AV block and only 10.8 % (+2.1) a in patient with complete AV block and heart dysfunction.