A seasonal difference in the temperature-pulse rate curve of the isolated frog's heart was described by Barcroft & Izquierdo (1931) . In the summer they found an almost linear relation between temperature and frequency of pulsation from 5 to 20 0 C, while in the winter the relation was exponential over the same temperature range. Carter (1933) investigated the effects of various endocrine and related substances on the form of the temperature-pulse rate curve. He found that the addition of

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... ine to the medium perfusing the heart transformed the curve given by the winter heart into the summer form, but had no effect on that of the summer heart. He concluded that the seasonal change in the temperature effect on heart rate was controlled by the endocrine system, and that the thyroid by increase in its activity in the summer was the effective agent. Although the change of frequency with temperature shows this seasonal difference, no experiments have been made to determine the transition periods between the two types of heart curve. Histological studies of the variations in the secretory appearance of the frog's thyroid have been made by Sklower (1925) and Meisenheimer (1936) and, if the agency effecting the alteration in the relation between heart rate and temperature is the thyroid hormone, it might be expected that seasonal data would provide physiological confirmation of the earlier histological observations. From September 1946, the form of the temperature-pulse rate curve of the isolated heart was determined on samples of frogs collected at monthly intervals throughout the year. In the course of this work it soon became apparent that the curves obtained could not be simply classified into the two types described by Barcroft & Izquierdo (1931) , but that other types were also appearing.-This rendered interpretation of their seasonal distribution in terms of thyroid activity impossible, as the factors leading to the appearance of the new types were not known. One of these new types of curve was assumed to be due to the uncomplicated action of temperature on the pulse rate, and was selected for further experimental work. Various endocrine preparations were added to the media perfusing such hearts, and it was found possible to reproduce the other types of curve which had been obtained from normal hearts. This paper presents a description of the various types of curve obtained in the seasonal survey, and their possible analysis in the light of these experimental results. Finally their seasonal distribution, particularly with reference to thyroid activity, is considered. jEB.28,2 10