1. A new technique, timed, quantitative neck suction was used to establish in normal man the temporal response patterns of the sinus node to brief baroreceptor stimuli. 2. Maximum sinus node inhibition occurred when stimuli were delivered about 0-75 sec before the anticipated appearance of the subsequent P wave; less pulse interval prolongation occurred when stimuli were delivered earlier, or later in the cardiac cycle. 3. The timing of maximum sinus node responsiveness appeared to be independent of base-line cardiac cycle length. 4. The duration of the baroreceptor-cardiac reflex are (the time from the onset of baroreceptor stimulation until the earliest occurrence of pulse interval prolongation) averaged 0-24 sec. 5. Pulse interval prolongation was proportional to baroreceptor stimulus strength and duration. 6. The results suggest that in man, sinus node responsiveness to baroreceptor stimuli depends critically upon the timing of stimuli within the cardiac cycle.