The time relation between the rise of arterial pressure provoked by phenylephrine and the resulting reflex bradycardia was examined in a group of young healthy subjects studied at rest and during exercise. When systolic pressure is related to pulse interval on a beat to beat basis during the pressure rise the best correlation is obtained if each beat is related to the pulse interval immediately following, provided the heart rate is less than 75/min. For faster heart rates the correlation is better if each pressure is related to the next-but-one pulse interval. Similar findings were obtained in a group of older subjects with higher arterial pressures. These results suggest that the conduction time of the whole reflex loop is about 775 msec, or 475 msec from the baroreceptors to the effect on the heart.