Summary.The rabbits have proved to be very sensitive to the toxic influence of the cod liver oil. Rabbits which were given a dose of oil of 5 cc. per kg. bodyweight per day have succumbed in 10 to 22 days and showed obvious alterations of their Ecg.In all the rabbits examined, morphological alterations of degenerative nature, spread throughout the whole heart muscle, have appeared.All the rabbits have exhibited alterations of their Ecg. when under the influence of the c.l.o., and these alterations may occur within a week after the initiation of the c.1.o. treatment.The electrocardiography alterations are in the main dependent on injuries of the function of the conduction system of the heart, which are also seen from prolongations of the a‐v conduction time and the QRS time. A prolongation of the latter time is usually accompanied with alterations of the appearance of the Ecg., in more pronounced cases of the same nature as in bundle branch block. If the animals survive this stage, the curve shows a tendency to regain more normal features, but its time relations remain increased. We have reasons for believing these conditions to be dependent on initially local alterations of the function of the conducting tissue, which later grow general.A prolongation of the duration of the whole ventricular contraction (prolonged Q—T‐time), which is considerably greater than the increase of the QRS‐time, seems to indicate, that the proper contracting muscle of the heart also reacts slower when under the influence of the c.1.o.In two cases, where it has been put to test, a larger dose of c.1.o. seem to be of less toxic effect, when the animal was previously accustomed to a small dose of oil.One animal, which received c.1.o. which had been irradiated with ultraviolet rays, remained remarkably little influenced.Green grass and summer sunlight — especially the bright spring day‐light — in these experiments also have shown a protecting influence on rabbits against the toxicity of c.1.o.The rabbits do not possess the same increased sensibility as the mice to the c.1.o when this is given emulated.An addition to the diet of vitamins B and C, in the form of marmite and lemon juice, does not protect the animals against the toxic influence