An experi-mental model for the study of nonobstructive pyelonephritis utilizing a virulent strain of E. coli and a technique for the collection of clean-voided urine specimens is described. Twenty-one experimental animals received approximately 100 million bacteria intravenously initially and 12 received an additional similar injection 4 weeks later. Eight rats served as uninjected controls. It appears that some strains of E. coli may persist for long periods in the urinary tract and that minute lesions may be responsible for bacteriuria.