The architecture of the mouse popliteal lymph nodes differs from that shown in conventional diagrams. The cortical lymphoid tissue, rather than forming a continuous outer layer, is organised into one or two hemispherical aggregates which project towards the hilus. These aggregates are surrounded by medullary tissue which thus extends to large areas of the surface of the node. The vascular distribution in the lymphoid aggregates is relatively sparse and contrasts with the dense meshwork of capillaries and venules around them. It also contrasts with the high vascularity of medullary tissue. Arterial vessels, especially those of larger calibre, are predominantly seen in the hilar area of the node suggesting that there is extensive branching as the artery enters the node. Capillaries associated with the lymphoid aggregates are usually lined by continuous endothelium, while those in the medulla are generally of the fenestrated type. The microcirculation has an extensive venous capacity and many venous segments are high endothelium venules whose walls are permeated by lymphocytes. Each node receives one or two afferent lymphatic vessels and is drained by up to four or five efferent lymphatic vessels. In approximately half the nodes examined, there were extranodal communications between afferent and efferent lymphatic vessels allowing some lymph to bypass the node.