The early pathologic changes in the kidneys of 4 infants dying from urinary infections consisted of edema, congestion and leucocytic infiltration of the intertubular tissue of the peripelvic areolar tissue and of the pelvic epithelium. Abscesses were found in the kidney and in the renal pelvis. The features of the healing lesion were diminution in the number of leucocytes, marked phagocytosis, the presence of polyblastic cells and of connective tissue. Late healing was represented by rather dense collections of leucocytes in regions of connective tissue. A description is given of experimental lesions of sterile hydronephrosis produced in rabbits. The location and the nature of lesions produced by intravenous injections of a localizing strain of colon bacillus are described in detail. Additional experimental lesions were produced by intravenous and intra-ureteral injection of organisms in animals in which the ureters were obstructed and in others in which the urinary tract was patent. The earliest lesions in the kidneys of the infants studied are comparable to the experimentally produced lesions of 3-5 days'' duration. Cortical abscesses are found in both, and are similar in appearance. Extensive involvement of the renal pelvis and peripelvic tissue was found in both and in experimentally produced lesions occurred in hematogenous as well as in ascending infections. It is to be particularly emphasized that in every instance both in human beings and in experimental animals in which early lesions occurred the pelvis participated in the process.