Shifts in the level of the atrial repolarization (P-Tp) segment caused by autonomic neural stimulation or by direct perfusion of either acetylcholine or norepinephrine into the sinus node artery were studied in 29 dogs. Direct perfusion of the atrioventricular (A-V) junction with acetylcholine (0.1 μg/ml), or vagal stimulation after direct perfusion of the sinus node with atropine (10 μg/ml), were the two methods utilized to produce transient complete A-V block without altering the sinus rate. In this fashion not only could the isolated P waves and P-T segments be examined, but also their influence on QRS complexes and S-T segments during normal conduction could be assessed in the same dog. Both cholinergic and adrenergic stimuli caused shifts of the P-Tp segment, but these shifts were more pronounced and consistent with use of either acetylcholine or stimulation of the vagus nerve. The right and left vagus nerves had opposite effects on the direction of shift of the P-Tp segment, thus indicating that asymmetry of vagal neural influence is a determinant of the magnitude of shift observed in any record of the P-Tp segment. From a comparison of tracings recorded both with and without A-V conduction, it is shown that an altered P-Tp segment may significantly distort both the QRS complex and the S-T segment, especially during A-V junctional rhythm. Since neural control of the heart is important in the pathogenesis of angina pectoris, arrhythmias and conduction disturbances, neurogenic influence on the P-Tp segment must be considered in evaluating the electrocardiographic changes in these conditions. © 1973 Dun · Donnelley Publishing Corporation 666 Fifth Avenue, New York, New York 10019 All rights reserved.