The RR interval and the partial conduction time in the electrocardiogram of vertebrates were studied from the viewpoint of comparative cardiology. They were important for the physiological or patho-physiological approach to the clarification of cardiac function. RR, PQ and QT intervals and QRS complex durations were measured on 245 electrocardiograms recorded from twelve vertebrate species. Cardiological data obtained from some other experiments and the results reported by other workers were referred to in discussion. Positive correlations were recognized in general among the partial conduction times in one another. However, positive correlations of RR-PQ, RR-QT and RR-QRS and a negative correlation on a bilogarithmic scale between the heart weight and the heart rate per minute in resting state were recognized only in homothermic animals. A relationship between the submaximal increasing ratio of heart rate and the heart rate in resting state was indicated approximately by a hyperbolic function. From the results obtained, it was presumed that the original pattern of the cardiac conduction system might be manifested in poikilothermic animals, and that animals showing a lower heart rate in resting state might be able to maintain a capacity of increasing the heart rate greater than those showing a high heart rate.