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IN THE PAST reciprocal rhythm in man has been associated mostly with AV nodal rhythm; there are several reviews of the lit- erature.1-4 In the described cases the AV nodal impulse activates first the ventricles, then after some delay the atria; somewhere in its course to or toward the atria the impulse presum- ably turns back to activate the ventricles again. This phenomenon was first recognized in electrocardiograms of man by White.5 Mines6 had previously observed a similar phenomenon in experimental animals, the im- pulse originating in an atrium or ventricle, traveling to the other chamber, then back to the chamber of origin, then back again to the other chamber, etc., etc. for a number of cycles. Pan7 applied this explanation to hu- man polygraphic tracings in which he believed the impulse originated in a ventricular pre- mature systole, traveled to the atria and then back to the ventricles. The phenomenon was possibly represented but not recognized in polygraphic tracings of Wenckebach.89 Re- cent studies indicate that reciprocal rhythm in man may be initiated by ventricular prema- ture systoles far more frequently than has heretofore been suspected and may often ac- count for the ventricular systole which follows the commonplace interpolated ventricular pre- mature systole.10