A simple method to perform serial renal clearance studies without urine collection in rats is described. This was applied to nonradiolabelled para-aminohippurate sodium (PAH) and iothalamate sodium (IOT) which were used respectively to estimate renal plasma flow (RPF) and glomerular filtration rate (GFR). Under isoflurane-anesthesia the jugular vein and carotid artery of Fischer 344 rats were cannulated, and a loading dose followed by a continuous infusion of PAH and IOT was administered. Steady state was reached after 30 min, and four arterial blood samples were collected over the next 30 min for analysis by HPLC (day 0). The clearances of PAH and IOT were calculated according to Fick's principle as estimates of RPF and GFR from the ratio of the infusion rates to the median solute concentrations. Three days later the femoral artery and vein were cannulated, and the same study was repeated (day 3). There was no significant difference in renal clearances of PAH and IOT between days 0 and 3. The described method gives values that compare well with others in the literature based on other methods and presents an accurate and simple way of detecting changes in renal function before and after a potentially nephrotoxic treatment regimen.