1. The metabolism of 50 microM [3-14C]coumarin has been studied in precision-cut-calf liver slices. 2. The metabolism of 50 microM coumarin to 7-hydroxycoumarin has also been examined in calf, rat, Cynomolgus monkey and human liver microsomal preparations. 3. In precision-cut calf liver slices, [3-14C]coumarin was metabolized to various polar products and to metabolite(s) that bound covalently to calf liver slice proteins. The polar products included 7-hydroxycoumarin (which was extensively conjugated with D-glucuronic acid and/or sulphate), metabolites of the 3-hydroxylation pathway (mainly o-hydroxyphenylethanol and o-hydroxyphenylacetic acid), and unknown metabolites. 4. Coumarin 7-hydroxylase activity was readily detectable in calf, Cynomolgus monkey and human liver microsomes, but only barely detectable in rat liver microsomes. Enzyme activity in calf, Cynomolgus monkey and human liver microsomes was inhibited by 8-methoxypsoralen (methoxsalen) with IC50's (concentration required to produce a 50% inhibition of enzyme activity) ranging from 0.3 to 2.8 microM. 5. These results and those of other studies demonstrate that precision-cut liver slices are a valuable in vitro model system for investigating species differences in xenobiotic metabolism. Coumarin is metabolized in calf liver by various pathways including both 3- and 7-hydroxylation. The inhibition of coumarin 7-hydroxylase activity by 8-methoxypsoralen suggests that calf liver microsomes contain P450A isoenzyme(s) similar to mouse 2A5 and human 2A6.