A challenge procedure based on reflux of Ureaplasma urealyticum to the rat kidney was used to examine the disease-causing potential of this species for the upper urinary tract. Renal infection was detected 8 days after challenge in 90 to 100% of animals exposed to 10(7) ureaplasmas but was not detected in animals exposed to fewer than 10(6) microorganisms. Viable microorganisms were recoverable from more than 60% of infected animals throughout a 24-week follow-up period of study. Animals with persistent infection exhibited a humoral response involving immunoglobulin M and immunoglobulin G antibody. These results show that U. urealyticum is capable of giving rise to chronic infection in the kidneys of rats following challenge via the ascending route.