The course of Lyme borreliosis in LEW/N rats inoculated intraperitoneally as infants with 106Borrelia burgdorferi was followed for 360 days. Spirochetes were detected in the blood through 30 days, in the brain through 60 days, and persisted in the spleen, liver, kidneys and articular tissue through 360 days. Acute exudative arthritis, tendonitis, and bursitis were evident in multiple joints by day 30. Arthritis regressed thereafter but capsular fibrosis and lymphoplasmacytic infiltrates persisted throughout the study. Several rats developed exacerbations of acute arthritis within days 180–360, a pattern similar to that encountered in human Lyme disease. Rats had a high prevalence of nonsuppurative myocarditis and vasculitis during days 90–360. Spirochetes were visualized by microscopy injoints and other tissues during the first month of infection, but were seen only sporadically thereafter. All rats seroconverted to B. burgdorferi by day 30. IgM titers persisted and IgG titers rose progressively through day 360. Immunoblots revealed IgM reactivity to a single 41 kDa protein until 360 days, when reactivity to a 60 kDa protein emerged. IgG reactivity occurred against progressively more proteins with time, indicating continued antigenic stimulation. Chronic and recurrent arthritic lesions and myocardial involvement suggest that the rat is a reliable model for further investigation.