The discharge characteristics of type B left atrial receptors were analyzed during alterations in heart rate. Recordings were made from single-fiber preparations of the left cervical vagus of pentobarbital-anesthetized, open-chest dogs. The heart was paced following a sinoatrial crush at frequencies ranging from 60 to 240 beats/min. Left atrial transmural pressure was varied at each heart rate by the intravenous infusion of warm isotonic NaCl. As heart rate was increased there was a progressive decrease in the level of peak “v” wave left atrial pressure. Concomitantly with the decrease in left atrial pressure, the number of spikes per cardiac cycle decreased as did the maximal instantaneous frequency of discharge. A significant positive relationship could be demonstrated with either the discharge per minute [(spikes per cycle) x heart rate] or discharge per cycle vs. the peak “v” wave of the left atrial pressure, regardless of heart rate. The number of impulses that entered the central nervous system per unit of time remained relatively constant at heart rates between 90 and 240/min. It is concluded from these data that the reflex effects which have been attributed in the past to atrial stretch receptor stimulation during clinical episodes of atrial tachyarrhythmias may be better correlated with some aspect of receptor discharge other than frequency or the number of discharges per cycle.