Adult male rats were subjected to pyelonephritis by direct kidney intramedullary injection of 0.1 ml saline suspension of 10(5) E. coli. Animals were killed at intervals of 4, 10, 15, 30 and 60 days. Half of each kidney and bladder were cultured in proper bacteriologic media to demonstrate the existence of infection. The other halves were submitted to light microscopy and ultrastructural studies. Immunofluorescence methods were used for the study of connective matrix components, at the initial stage of the inflammatory process (4, 10 and 15 days). Infection was documented by bacteriologic, gross and microscopic findings in all groups following inoculation, and it lasted up to two months. Following the acute inflammatory reaction, fibronectin and type III collagen were deposited in the interstitium of kidneys. Small amounts of type I collagen were found later. Type IV collagen appeared in small quantities, associated with collapse of structures containing basement membranes. Fibronectin became concentrated in re-activated foci. The subsequent scarring and associated focal renal atrophy depended upon the extension of the acute lesion.