Abstract: Experimental ascending acute pyelonephritis was produced in rats by injecting 0.5 ml of 10⁶/ml or 10⁸/ml Escherichia coli O111B4 into the urinary bladder via. without unilateral ligation of the ureter. Results of the research showed the pathologic changes of kidneys were not so serious and no purulent were found,but the process of inflammation was typical.Two groups of the Escherichia coli O111B4 all could produce acute pyelonephritis and did not show any side preference.