Summary.The anatomical lesions produced by the c.1.o in the hearts of the rats in this as in previous investigations (Aguirre) is found to be very small, when compared with the lesions occurring in the hearts of the mice. They are, however, in part of about the same nature its these and injuries are observed in all those animals examined, which received the oil.In spite of the comparatively small heart lesions the rats succumb, most certainly on account of the c.l.o. treatment, at an earlier stage of the experiment than the mice, and rats getting greater doses die earlier than those getting smaller. No rat in these experiments has reached the age of 10 months.The Ecg. of the normal rat differs in its common appearance from the Ecg. of the mouse, but comes ntw to the outlines of the Ecg. in man. During the first period of the post natal growth the Ecg. of the rat is subjected to alterations of the same kind as the Ecg. in children. Thus the electrical axis in the rats during the first months of life turns in an anti‐clockwise direction, from the age of one to that of two months by about 20°.In rats treated with c.l.o. the alterations in the Ecg. are very small and even the enormous dose of oil of 12 cc. kg./day given for more than 5 months did not produce appreciable alterations.The greatest alterations observed occur in the animal treated tlie longest time (9 months), although it got the smallest dose of oil (0.5 cc. kg./da