Pyelonephritis was studied after an intravenous injection of Candida albicans, Staphylococcus aureus , or enterococcus in alloxan-diabetic rats and in water-diuresing or non-diuresing nondiabetic rats. The renal microbial populations of C. albicans or S. aureus were found to be > 10 super(5) colony-forming units per g for up to 42 days in diabetic rats, whereas the kidneys tended to become sterile in nondiabetic rats. No significant difference was found in the course of enterococcal pyelonephritis in diabetic versus control rats. The studies demonstrate greater severity of infection in the diabetic kidney due to S. aureus) and C. albicans , which can be partially explained by decreased inhibitory activity of urine for these organisms in diabetic rats.