In rats with glycerol-induced acute renal failure (ARF) and controls, renal concentrations of 125I-labelled sodium diatrizoate were measured 5 min after intravenous doses ranging from 14 to 1,800 mg/kg body weight. Renal concentrations of diatrizoate, at all doses, were much higher in controls than in ARF. A linear relationship existed between dose and renal concentration in ARF. In controls, at doses above 225 mg/kg body weight, the fraction of the dose present in the kidneys diminished and renal iodine content approached that observed in ARF. Differences in diatrizoate concentration which existed between cortical and medullary zones in healthy kidneys at low doses were progressively eliminated as dosage increased, consistent with the osmotic diuretic effect of diatrizoate. In ARF, the pattern of intrarenal distribution at all doses was similar to that seen in controls at high doses, though concentrations in outer cortex were consistently slightly higher than those in inner cortex. These observations suggest continuing, though reduced, filtration of diatrizoate in ARF by glomeruli already subjected to a large solute load.