The vascular and stromal arrangements of the red pulp in congested and contracted dog spleens were studied by transmission electron microscopy. Each dog had been injected intravenously with Thorotrast to label actively endocytizing cells. Only macrophages ingested Thorotrast. The proximal portion of each arterial capillary was surrounded by a "periarterial macrophage sheath" (PAMS), a term we introduce to replace the term "ellipsoid". PAMS were composed of a fine meshwork of reticular cells and reticular fibers which held tightly-packed macrophages and interspersed blood cells. These macrophages, as well as those in the reticular meshwork of red pulp, contained Thorotrast, cell debris, and deposits of hemosiderin. The arterial capillary at the center of each PAMS was formed by parallel, rod-shaped endothelial cells and discontinuous layers of basement membrane and reticular-cell cytoplasm. PAMS were tapered at their distal ends; the terminal portion of the arterial capillary continued beyond the PAMS to end in the reticular meshwork of red pulp. Endothelial cells in the terminal arterial capillaries were separated by gaps through which blood cells passed into the spaces of the reticular meshwork of red pulp. The reticular meshwork was formed by reticular cells which appeared to be specialized for contraction. These cells were filled with thin filaments and possessed plasmalemmal dense bodies as found in smooth muscle cells. Furthermore, the reticular meshwork was innervated by unmyelinated adrenergic axons which probably were derived from nerves that followed arterioles. Axons were enclosed in surface invaginations of cells which were similar to reticular cells in shape and cytologic detail and which we called "axon-bearing reticular cells". Axon-bearing reticular cells were inserted between the branches of the reticular cells that formed the meshwork. Venous sinuses formed an anastomosing system of vessels draining into pulp veins which then joined trabecular veins. Sinuses were formed by parallel, rod-shaped endothelial cells encircled by strands of basement membrane and reticular-cell branches. Endothelial cells lay closely side by side except where interendothelial slits were opened by blood cells passing into the lumen or by pseudopodia of macrophages which lay outside the sinus. Cell traffic across the sinus wall was greatest in areas where blood cells were mixed with plasma. Congested spleens stored concentrated red cells in both sinuses and the reticular meshwork; contracted spleens were emptied of blood. The reticular meshwork may contract to assist trabecular and capsular smooth muscle in expelling stored red cells and effecting hemoconcentration.