Retrograde pyelonephritis was produced in rats by introducing into the bladder a small refluxing inoculum of Escherichia coli that would enter the renal pelvis but not the blood stream. At the same time the left ureter was partially obstructed for 18 hours. This model differs from previous attempts to produce E coli pyelonephritis with large (0.6 ml) volumes infused into the bladder, because such large volumes cause bacteremia and hematogeneous pyelonephritis. Since retrograde E coli pyelonephritis in patients is not accompanied by positive blood cultures, the model described in this report is believed to accurately mimic human pyelonephritis and to allow a realistic approach to the study of immunity against retrograde infection in the urinary tract.