In a fundamental study on the kinetics of the urinary excretion of a number of agents secreted by the renal tubules of the mammalian kidney, we measured plasma levels of iodopyracet after intravenous application to male beagle dogs. The animals were anesthetized throughout the experiments with sodium pentobarbital, and provisions were made for the collection of blood and urine samples. In order to obtain a sufficiently high urine flow throughout the experiments, the dogs received a constant infusion of a solution containing 5% mannitol and 0.5% inulin. Inulin was added for measurement of the glomerular filtration rate. Linear plots of the urinary excretion rate against the average plasma concentration of each urine collection period were drawn as an illustrative way to depict the relation between plasma concentration period were excretion rate. For the mathematical description of these so-called tubular titration curves, a dynamic model was conceived to characterize the various renal transport mechanisms. Parameters for the description of the renal handling of iodopyracet were estimated by a simulation procedure with the aid of the computer program CSMP III.