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The excretory behavior of nine nephrotropic contrast agents with varying physicochemical properties such as charge, lipophilicity, and molecular size was investigated. Renal clearance in comparison with inulin was determined by means of the continuous infusion method. Each contrast agent was infused at three dose levels in four to six rabbits. The investigations show that tubular transportation in proportion to glomerular filtration decreases with increasing dosages of all the contrast agents. Thus, with the highest concentration in plasma all contrast agents are eliminated at more or less the glomerular filtration rate (GFR). After administration of the low dosages the following differences are found: 1) Net tubular secretion increases for the monomeric contrast agent acids with increasing lipophilicity, in the order diatrizoate congruent to iothalamate less than iodamide less than acetrizoate. 2) The clearance studies do not reveal any tubular secretion or reabsorption for a hydrophilic cationic contrast agent. 3) The nonionic contrast agents do not show net secretion. The more lipophilic they are, the more they are reabsorbed. 4) Two dimeric contrast agents also do not reveal any tubular secretion. They seem to be reabsorbed more than monomers with the same charge.