A cardiotachometer has been devised to record the variation of the frequency of the sino-atrial node, but no apparatus has been reported which simultaneously records the frequency variations in atrioventricular conduction time and the sino-atrial node frequency. In an attempt to demonstrate how the sinus rhythm affects the A-V nodal conduction time, excitation conduction time between the right atrium and ventricle was recorded continuously by a newly devised A-V interval meter and compared with the variation in the length of the cardiac cycle (R-R interval).In the post-operative unanesthetized dog, the respiratory variation in A-V interval was from 7 to 13 msec while the R-R interval ranged from 350 to 500msec. The variations of the A-V and R-R intervals were in phase under nervous control but out of phase with pacing and pharmacological intervention. Both variations diminished in amplitude when the dog stood up or ate, when propranolol was administered intravenously, or when atropine was given.In the anesthetized animal right vagal stimulation caused an abrupt lengthening of the R-R interval after one second or less, while left vagal stimulation usually prolonged the A-V interval after more than one second. Stimulation of the left stellate ganglion resulted in a shorter A-V conduction time without a change in the R-R interval, while stimulation of the right stellate ganglion generally caused tachycardia.