Escherichia coli-induced pyelonephritis was studied in untreated alloxan-diabetic rats, insulin-treated diabetic rats, glucose water-drinking (diuresing) nondiabetic rats, and tap water-drinking (nondiuresing) nondiabetic rats following injection of E. coli either into the emptied urinary bladder, into the left kidney, or intravenously. For prevention of an ascending infection in the right kidney, the right ureter was ligated and transected immediately prior to bladder or intrarenal inoculation. These experiments established that in normal rats ascending renal infection alone occurred following introduction of small inocula into the bladder--and then only when facilitated by diuresis. In diabetic rats both ascending and hematogenous renal infection occurred following introduction of small inocula into the bladder. Insulin treatment that reduced hyperglycemia also reduced glycosuria and restored urinary antibacterial activity against small inocula of E. coli but only partially reduced polyuria and prevented hematogenous but not ascending infection. Thus, hyperglycemia was probably the major factor promoting hematogenous renal infection, whereas polyuria--and therefore vesicoureteral reflux--was the major factor promoting ascending infection.