The urinary bladder is especially subject to infection by virtue of its direct connection to the external urethral opening, and it is natural to anticipate the presence of a well-developed immunological mechanism to respond to this potential threat. The present study describes small, very highly vascular lymph nodes located in the wall of the rabbit bladder, which may be involved in a local response to foreign antigens. The vasculature and structure of these lymph nodes was described using a combination of vascular corrosion casting, ink injection, and light and electron microscopy. The distal abdominal aorta was cannulated, and after clearing the bladder vasculature with buffered saline, one of the following procedures was used: 1) the bladder was perfuse-fixed in preparation for light and electron microscopy; 2) the bladder vasculature was filled with India ink for vessel tracing; or 3) vascular corrosion casts of the vasculature were prepared by infusing resin comprised of a mixture of Mercox, methyl methacrylate monomer, and catalyst. The resulting casts were cleaned with KOH, formic acid, and water in preparation for scanning electron microscopy. Vascular casts and India ink injections revealed the presence of a number of isolated capillary tufts consisting of clusters of one to five ''glomeruli,'' closely associated with the major vesicular vessels along the lateral walls of the bladder, and supplied by tertiary branches of these vessels. Light and electron microscopy showed that the capillary tufts represented the blood supply to small, ovoid lymph nodes located near the serosal surface of the bladder wall and usually restricted to the basal half of the bladder. These nodes were encapsulated and exhibited subcapsular sinuses, numerous small blood vessels, a limited number of high endothelial cells, and, occasionally, nerves and a follicular substructure. The nodes contained abundant lymphocytes, stellate stromal cells, macrophages, and eosinophils, but lacked the obvious cortical and medullary organization and germinal centers often seen in larger lymph nodes. Vascular corrosion casts, vascular ink injections, and microscopic examination confirmed the presence of small, highly vascular lymph nodes closely associated with the main vesicular vessels along the lateral walls of the rabbit bladder. A follicular substructure of the nodes appears to correspond with the ''glomerular''