Aprotinin (Ap), a basic polypeptide with a molecular weight of 6500, is filtered at the glomerular membrane without steric restriction and is completely absorbed by the proximal tubule cells. Here Ap is broken down to amino acids, but no breakdown products enter the peritubular circulation during the first 20 min following an intravenous injection. These properties have recently been exploited for measurement of local glomerular filtration rate, based on the assumption that the proximal tubular uptake site is located at the level of the filtering glomerulus. To evaluate that assumption we have now made serial autoradiographs of the rat kidney 20 min after intravenous injection of 2-750 mug of I-125-Aprotinin. With all doses the percent I-125-containing proximal tubular transections were about 50 in the outer and middle cortex and 35 in the inner third. We interpret these numbers to mean that all filtered Ap is taken up in the first two thirds of the proximal convoluted tubular length and does not reach the pars recta. Since the proximal tubule on average is located more superficial than its glomerulus, measurement of local Ap uptake will tend to overestimate glomerular filtration rate in outer layers of the cortex. Quantitative estimate of this "displacement" will be presented in a companion article. (C) 2003 Elsevier Science (USA). All rights reserved.