The structure of the sinus walls in the popliteal lymph node of the rabbit was studied with the electron microscope. In the marginal sinus, the endothelial cells are connected by gap junctions, puncta adherentia, and surface specializations characterized by focal approximation of the adjoining membranes without fusion. They possess large numbers of simple and compound uncoated invaginations of the plasma membrane that are closed by a diaphragm with a central thickening. The tissue strands that straddle the lumen of the sinus consist of a fibrous core containing both collagen and elastic fibers, surrounded by endothelial cells identical to those composing the outer sinus wall. Cortical sinuses that run independently of the trabeculae were identified by exploiting the fact that their endothelial cells accumulate lymph-borne ferritin, and their lumen is outlined by horseradish peroxidase administered intravenously. They are lined by a flattened, continuous endothelium and lack luminal strands. The walls of the medullary sinuses consist of endothelial cells and macrophages. The endothelial cells are interconnected by specialized junctions and contain fewer plasmalemmal vesicles than in the cortex; furthermore, dense granules are present in their cytoplasm. Macrophages adhere to the surface of the endothelial cells; typically, none of the junctional specializations that characterize the interface between endothelial cells connect endothelial cells to macrophages. However, at points along the contact region with the endothelium, the plasmalemma of the macrophage is decorated by an attachment plaque of fluffy cytoplasmic material. Sinus endothelial cells slowly accumulate lymph-borne ferritin like vascular endothelial cells elsewhere in the body, whereas macrophages contain both ferritin and engulfed erythrocytes.