Seven hamsters, experimentally infected with Borrelia burgdorferi, were examined by both cultural and histological techniques at 1 to 9 months postinfection. Spirochetes were detected in the spleen, kidney, or eye of all animals by culture and in the spleen, kidney, eye, liver, or heart blood of five of seven animals by histological examination. Two animals showed nonspecific hepatic portal lymphocytic infiltration, while five of the hamsters displayed no significant histologic signs of inflammation or granuloma formation in the major organ systems. Synovitis and arthropathy did not occur. All animals showed some degree of follicular lymphoid hyperplasia of the spleen. Spirochetes were predominantly extracellular with a rare organism appearing to be partially within a macrophage.