Atrial contribution to ventricular performance was assessed by a non-invasive photoplethysmographic technique in two patients with complete atrioventricular block. The amplitude of the dorsalis pedis pulse volume showed a parabolic relationship to the P-Q interval at ventricular pacing rates ranging between 51 and 103/min. The optimal P-Q intervals were inversely related to the ventricular rate. The interval between the onset of the QRS complex and the foot of the dorsalis pedis pulse wave, Δt (Q-f), also showed a parabolic relationship to the P-Q interval, but the changes in Δt (Q-f) were opposite to those in pulse amplitude. Optimal P-Q intervals were associated with shortest Δt (Q-f) and highest pulse volume amplitudes at all ventricular rates. Pulse wave velocity along the femoralis-dorsalis pedis segment was constant and independent of the P-Q interval and of variations in pulse volume amplitude.