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The authors present the data of the histological structure of the dog heart tissue adjacent to the electrode tip following a ten-month period of electrical stimulation by means of an implanted graphited porous endocardial electrode and a pacemaker of the alternating polarity. The reason for performing such an histological investigation was an extremely low chronic threshold of heart electrostimulation, amounting to 0.35 V with the stimulus duration 1 ms. The structural features of the tissues as viewed by the authors, consist in (1) the thinning of the fibrous capsule of the electrode tip; (2) the presence of a zone adjacent to the electrode, with this zone being rich in the liquid component of the intercellular substance; (3) the formation of a conspicuous radix of the fibrous capsule, penetrating into the depth of the myocardium.