THE LYMPHATIC VESSELS OF THE SINU-ATRIAL (SA) node were investigated after injection of Indian ink plus gelatine in 62 canine and 31 human hearts, the latter of subjects aged between 21 and 80. Lymph was found to be chiefly drained from the SA node of the dog by a vessel which collected lymph near the left margin of the SA node and ran between the superior vena cava and the right circumference of the aorta into the left cardiac lymph collector and through it into the left cardiac node. Approximately in two thirds of cases this main lymph drainage was supplemented by smaller lymphatic branches running into the right traceo-bronchial nodes. In man, the main route of lymph drainage from the SA node is represented by one or two vessels which run to the right and upwards along the superior vena cava into the inferior tracheo-bronchial nodes (of the bifurcation), lying near the right bronchus. Little branches draining lymph into the cardiac collector are found rarely. It has become evident from histological sections that the lymphatic vessels run between the cells of the conduction system and the collagenous fibres of connective tissue and are not numerous.