Unilateral pyelonephritis was induced in 50 rabbits by injecting Escherichia coli (minimum inhibitory concentration of tobramycin 0.25 mg/l) into the left kidney and by obstructing the ureter temporarily. Tobramycin treatment (daily dose 10 mg/kg) was started 4 days after surgery, either in a single daily dose or in 3 divided doses at 8 h intervals, for 2, 3, 5, 7 or 10 days. Comparison of bacteriology, renal morphology, and renal functions (BUN, serum creatinine, alanine aminopeptidase, N-acetyl-beta-D-glucosaminidase, cathepsin B, sphingomyelinase) suggests better efficacy and renal tolerance of the single daily dose regimen in the treatment of experimental acute pyelonephritis.