Iodopyracet (Diodone) is a radiocontrast agent used in urography before 1950. Renal clearance of iodopyracet is characterized by supply-limited elimination at low plasma concentrations and capacity-limited elimination at high plasma levels. Iodopyracet to be an effective agent for the estimation of renal plasma flow and tubular function has been used extensively in physiological studies. In 1945 was found, that p-aminohippuric acid was in some ways superior to diodone for these estimations in man because the urine and plasma blanks are small and because diodone penetrates human red blood cells whereas p-aminohippuric acid does not.