The ultrastructure of the ureter in the nonhuman primate is described. In experimentally produced chronic pyelonephritis, the ureter shows extensive ultrastructural changes throughout the wall. The epithelium has cellular damage progressing from the basal cells to the superficial cells. There is an invasion of leukocytes into the intercellular spaces at all levels. The connective tissue layer seems thickened with an increase in active fibroblasts, leukocytes, collagen, and elastic fibers. There are various stages of damage in the smooth muscle layer and an abnormal increase of connective tissues between bundles and smooth muscle cells. The interdigitating nexuses seem stretched and altered. The changes are similar to those seen in ureteral obstruction and are assumed to be reparative.