The kinetic parameters of the cytostatic agent 5-fluoro-2'-deoxyluridine (FUDR) were studied in isolated rat hepatocytes and in the isolated perfused rat liver. In both experimental setups a dose dependency of the elimination parameters, half-life and clearance, was observed with a calculated turning point around 250 microM. In the medium of rat hepatocytes incubated at low (0.1 microM) to high (2000 microM) FUDR, the majority of the metabolites consisted of the catabolite alpha-fluoro-beta-alanine. The nucleobase metabolites, 5-fluorouracil and its primary product 5,6-dihydro-5-fluorouracil, approached apparent steady-state levels comprising 10 to 15% of the initial concentration. In the intracellular phase of hepatocytes incubated at 300 microM FUDR almost 90% of the FUDR-derived material was alpha-fluoro-beta-alanine, whereas essentially no unchanged FUDR could be detected. Similar results were obtained at extracellular FUDR concentrations exceeding 300 microM. In the isolated perfused rat liver, the clearance decreased to 15 to 20% of the corresponding values when the initial concentration was raised from 24 to 2400 microM. At the end of perfusion alpha-fluoro-beta-alanine comprised 90 to 95% of FUDR-derived total radioactivity in the tissue even at initially 2400 microM FUDR, although at this FUDR dosage 20% of the substrate remained unmetabolized in the medium. These results suggest that the limitation of hepatic FUDR elimination is not due to saturable hepatic metabolism but must be due to saturable uptake of these pyrimidine derivatives across the cellular membrane of parenchymal liver cells.