In the isolated rabbit hearts the presence of an electrical longitudinal dissociation in the atrial portion of the His bundle was examined by the microelectrode method. Under the normal condition no marked differ-ence was found in the duration of the action potential of the cells at the same transverse level of the His bundle. However, the possibility of separation to at least dual pathways of the His bundle was disclosed, when the arrival time of excitation was observed or when, in addition, the site of stimulation was changed or intracellular stimulation was applied.
By passing the current intracellularly into the His bundle the elec-trotonic potential was found to be large in a direction parallel to the running of the His bundle while it was found to be almost zero in a direc-tion perpendicular to it. The longitudinal electrical separation was pre-sumed, however, not to be complete under the normal condition, because the intracellular stimulation of one tract was found to be transmitted to the other tract and the spontaneous activity of one tract was found to be conducted to the other tract. A high potassium content or low sodium content did not increase the longitudinal separation markedly in most cases. But in 1 experiment a high potassium content increased it, showing a possibility that under abnormal conditions a complete longitudinal electrical dissociation may occur.