Coumarin, a well recognized rat hepatotoxicant, also causes acute, selective necrosis of terminal bronchiolar Clara cells in the mouse lung. Further, chronic oral gavage administration of coumarin at 200 mg/kg, a dose that causes Clara cell death, resulted in a statistically significant increased incidence of alveolar/bronchiolar adenomas and carcinomas in B6C3F1 mice. In contrast, mouse lung tumors were not observed at the 100 and 50 mg/kg dose levels in the oral gavage study, or in CD-1 mice following chronic intake of coumarin at levels equivalent to 276 mg/kg in diet. The current studies were designed to determine the impact of oral gavage vs dietary administration on the pharmacokinetics and metabolism of coumarin in CD-1 and B6C3F1 mice and F344 rats. Following the administration of 200 mg/kg 14C-coumarin via oral gavage, lung C(max) values (total 14C-associated radioactivity) were five- and 37-fold greater than those resulting from a 50 mg/kg oral gavage dose or 1000 ppm in diet, respectively. Coumarin (200 mg/kg) pharmacokinetics and metabolism was also examined in F344 rats following oral gavage dosing. Total 14C-coumarin associated radioactivity in plasma was 3.5-fold lower than in the mouse, and the plasma half-life in rats was five-times longer than in mice. Using non-radiolabeled compound (200 mg/kg), coumarin and products of the coumarin 3,4-epoxidation pathway were quantitated in plasma and urine after oral gavage administration to mice and rats. 7-Hydroxycoumarin (7-HC) was quantitated in mouse plasma and urine. o-Hydroxyphenylacetic acid (o-HPAA) reached a concentration of 37 microg/ml in plasma, and accounted for 41% of the dose in the urine, whereas the C(max) for 7-hydroxycoumarin was 3 microg/ml, and represented 7% of the administered dose. In the rat, the plasma C(max) for o-HPAA was 6 microg/ml, and accounted for 12% of the dose. The coumarin C(max) in rat plasma was comparable to that in mouse. Coumarin 3,4-epoxide (CE) and its rearrangement product o-hydroxyphenylacetaldehyde (o-HPA) and o-hydroxyphenylethanol (o-HPE), were not detected at any time point in plasma or urine. This analysis of coumarin and CE pharmacokinetics in rodents suggests that the differential tumor response in the mouse oral gavage and dietary bioassays is a function of the route of exposure, whereas species differences in lung toxicity between mice and rats result from heightened local bioactivation in the mouse lung.