Gerbils appear to be susceptible to infection by human isolates of Borrelia burgdorferi; we obtained 100% infection. Isolation of the B. burgdorferi from different organs six months post infection causes a generalized infection thus demonstrating that borreliae persist in these animals for a long period. Spirochetemia was present for 14 days, apparently in two intervals. The Borrelia burgdorferi specific antibody titers increased with time after infection thus indicating the persistence of spirochetes. The intraperitoneal inoculation of the B. burgdorferi to six gerbils of groups A and B induced significant histopathologic changes in most of the major organ systems and their surrounding adipose and fibrous connective tissues. The infiltrates consisted mainly of lymphocytes and histiocytes. Various numbers of plasma cells, eosinophils and high numbers of mast cells were also present. Three further animals which served as controls displayed no histological signs of inflammation in any organ system. No significant differences were noted between the histopathological findings seen in the animals of groups A and B (infected with cells from subcultures no. 25 and with no. 5, respectively). The persistence of B. burgdorferi and the high number of organs involved with slight to severe signs of inflammation in this series can be compared to persistence and to the multiorgan involvement seen in human Lyme disease. Thus gerbils can serve as suitable experimental animals to study the pathogenesis of Lyme disease and the extent of organ damage caused by B. burgdorferi.