Using for the delivery of current a section electrode sucker the authors have investigated the distribution of the electrotonus on the surface of the atria, the ventricles and the pseudotendons of dogs and cats. In all the structures studied there was more or less marked electrotonic anisotropy. The electrotonus spread along the fibres over considerably greater distances than in the transverse direction. The heaviest anisotropy was observed in the pseudotendons and the least in the specialized surface layer of the ventricle. The authors have determined the constants of attenuation on the surface of the cardiac syncytium along the fibres and in the transverse direction. From the findings they evaluated the "true" values of the length constants of the cardiac fibres. A qualitative comparison is made of the "number" of lateral contacts between the cells of the different cardiac structures.