1. Carotid baroreceptors of eight healthy young men were stimulated with brief (0.6 sec), moderate (30 mmHg), single or repetitive (2, 3 or 5), R wave coupled neck suction during held expiration, and brachial arterial pressure and pulse interval changes were measured for 10 sec. 2. Mean systolic arterial pressure declined significantly from control levels after single or multiple baroreceptor stimuli. The reduction of systolic pressure was significant within the first sec after the onset of trains of baroreceptor stimuli; its duration was proportional to the number of stimuli. 3. The sinus node response to a single baroreceptor stimulus was biphasic: inhibition was followed by less intense, but more prolonged facilitation (cardio-acceleration). A single baroreflex stimulus modulated sinus node function for about 7 sec. The integral of sinus node inhibition was linearly related to the number of baroreceptor stimuli. Inhibition with five repetitive stimuli peaked at about 1.5 sec after the onset of stimulation and delcined to a new steady-state level during the period of baroreceptor stimulation. 4. Human baroreflex adjustments of arterial pressure and sinus node function occur very early after a step increase of phasic baroreceptor afferent activity. The perturbation of sinus node function is complex and probably reflects changing temporal relationships between the arterial pulse and sinus node activity, and interactions between oscillating levels of acetylcholine and sinus node responsiveness to acetylcholine.