The chronotropic responses to repetitive bursts of vagal stimulation were determined in open-chest, anesthetized dogs. Either 5 or 10 electrical pulses were included in each stimulus burst, and the interpulse interval (Δ) was varied over the range of 5 to 150 msec. As the frequency of the stimulus bursts was progressively changed, the sinoatrial (SA) nodal pacemaker cells became synchronized with the repetitive bursts of stimuli over a certain range of burst frequencies. The magnitude of this frequency range varied with Δ. For 5 and 10 pulses/burst, the values of Δ that produced the greatest magnitude of this frequency range were 30.2 and 24.3 msec, respectively. Also, over the range of values of Δ from 5 to 50 msec, the magnitude of the negative chronotropic effect of the vagal stimulus burst varied directly with Δ It is likely that, as the interpulse interval is increased within this range of values, either more acetylcholine is released from the vagal nerve endings per pulse or there is less saturation of the receptors on the pacemaker cell membranes during each burst.