The QRS complex in the ECG of poikilotherms is increased by bleeding. The opposite reaction is observed in homeotherms. In previous studies, the type of QRS response to hemorrhage and infusion was ascribed to a phylogenetic difference, namely the presence or absence of a specialized conduction system in the heart. In order to substantiate the role of the specialized conducting system, two mammalian preparations were designed using the same species (cat) under conditions allowing both specialized conduction and cell-to-cell conduction. Comparison of the changes observed during bleeding or transfusion in the R’ wave of cats with bundle branch block, and in the depolarization wave obtained by ventricular pacing (both of which mimic poikilotherm conduction), with the changes in the R wave in normal cats, clearly show the dependence of the changes in QRS amplitude on the type of conduction system in the heart.