The purpose of this study was to contrast the effects of static and pulsatile pressure on carotid baroreceptor activity over a wide range of mean arterial pressure. Static and pulsatile pressure were applied to the isolated carotid sinus of dogs anesthetized with chloralose. Recordings were obtained from single baroreceptor units as well as from the whole sinus nerve or a large strand of the nerve. Three observations are reported. First, in single units the pulsatile pressure threshold, which averaged 48 ± 8 (SEM) mm Hg, was far below the static pressure threshold, which averaged 79 ± 8 mm Hg (p<0.05, n=15). Thus, pulsatility decreased the threshold by an average of 31 mm Hg in contrast to the minimal or lack of decrease in threshold reported by others in aortic baroreceptors. Second, at moderate arterial pressures a shift from static to pulsatile pressure caused a decrease in single and multiple unit activities. In single units, the decrease approximated 15% (from 42.0 ± 2.1 to 35.5 ± 1.9 spikes/sec, p<0.05, n = 25). In all units, there was no diastolic nerve activity (“silence”) when diastolic pressure was 1 to 10 mm Hg above static pressure threshold; 80% of the units exhibited "diastolic silence" when diastolic pressure was 20-30 mm Hg above threshold and 40% of the units showed silence at-diastolic pressures 40-50 mm Hg above threshold. In whole nerve recordings, pulsatility increased activity from 57 ± 15 to 142 ± 29 spikes/sec (p<0.05) at low mean arterial pressures (50 and 75 mm Hg), as expected from the reduction in pressure threshold noted in single units, and decreased activity approximately 15% (from 373 ± 69 to 320 ± 55 spikes/sec, p<0.05, n = 9) at mean arterial pressures of 125 and 150 mm Hg. This decrease in activity with a shift from static to pulsatile pressure at moderate arterial pressures has not been reported previously. Third, the static pressure-activity curve was sigmoid, and its gain peaked sharply at 75-100 mm Hg; in contrast, the pulsatile pressure-activity curve was linear between 25 and 150 mm Hg, and its maximum gain was half the maximum gain during static pressure. These differences between the static pressure-activity curve and the pulsatile pressure-activity curve were noted during both increases and decreases in carotid sinus pressure; both curves exhibited some hysteresis during the decreases in pressure. (Circulation Research 1987:61:648-658)