The metabolism of coumarin and 4-methylcoumarin by rat-liver microsomes has been studied. The major metabolites of coumarin in vitro, viz. 3- and 7-hydroxycoumarin, o-hydroxyphenyllactic acid, and o-hydroxyphenylacetic acid, were identified by thin-layer chromatography, and ultraviolet and fluorescence spectroscopy. Metabolites derived from 4-methylcoumarin in vitro have shown similar chromatographic and spectral characteristics as those of coumarin, suggesting that they are the methyl analogues of corresponding coumarin metabolites.