The lymphatic vessels of the ventricular conduction system, i. e. the atrioventricular (AV) node, AV bundle and bundle branches, were studied after injecting a series of 34 human and 50 canine hearts with India ink suspended in gelatin. In man, the lymphatic vessels of the AV node and the upper part of the AV bundle open into the subendocardial lymphatic network at the point of insertion of the septal cusp of the tricuspid valve; from there, the lymph is drained in part into the posterior branch of the left lymphatic trunk under the coronary sinus and in part across the membranous septum through the right atrial wall upwards into the inferior tracheobronchial nodes. The lymphatic network of the bundle branches is represented by a subendocardial network whose draining lymphatic vessels run ventrally and dorsally through the interventricular septum opening into the anterior and posterior branches of the left cardiac lymphatic collector. Anastomoses between the lymphatic vessels of the branches and those of the AV node through the AV bundle are rare. The lymphatic drainage of the conduction system of the canine ventricles is essentially similar to the lymphatic drainage encountered in man. The only difference between the two species is that in man the lymphatic vessels draining the AV node run ventrally towards the inferior tracheobronchial nodes, whereas in the dog these vessels open into the right cardiac lymphatic collector.