Hydronephrosis and simulated intrapelvic calculi were produced in laboratory animals. Cultures of proteus and E. coli were introduced in these kidney preparations through an extremely fine (PE 10) polyethylene catheter. Massive doses of appropriate antimicrobial agents could not clear the infection and the kidney progressed to pyelonephritic atrophy. Conversely, infection could not be maintained in obstructed kidneys or if the calculus was removed before the pyelonephritic changes became irreversible.