Homozygous and heterozygous female Gunn rats show increased susceptibility to experimental urinary infection. The strain develops pyelonephritis after intravesical inoculation of Proteus mirabilis in numbers which fail to induce the lesion in albino rats, and severe pyelonephritis is frequently complicated by papillary necrosis. The basis for this enhanced susceptibility has not been defined, but the occurrence of the phenomenon in both homozygous and heterozygous rats indicates that it is not caused primarily by high plasma levels of unconjugated bilirubin or by the deposition of bilirubin in the tip of the renal papilla. The increased susceptibility of the homozygous Gunn rat to ascending urinary tract infection provides supporting evidence for the suggestion that infection may complicate the natural history of experimental analgesic nephropathy in this strain and is relevant to the clinical association of analgesic nephropathy and urinary infection.