Abstract: An experimental chronic pyelonephritis（CPN） model was established by challenging the infant female rats with Escherichia coll. O111B4. On the basis of previous bacteriological examinations, histopathological and immunohistochemical studies were carried out simutanuously. Slight histopathological changes were observed in the infected kidneys during the chronic period, which resembled covert CPN in human. Positive-stained substances, as demonstrated by double bridge-PAP, were found not only in the focal areas, but also outside as seen within the normal epithelial cells, the morphology of which was quite similar to that of L-forms. Thus it is possible that L-forms play some roles in the pathogenesis of CPN.