A major novel coumarin metabolite was isolated from rat hepatic microsomal incubations by high-performance liquid chromatography. In the presence of a rat liver cytosolic fraction and NADH it was rapidly metabolized to O-hydroxyphenylethanol. The metabolite co-chromatographed with an authentic sample of O-hydroxyphenylacetaldehyde and its identity was confirmed by mass spectral analysis. The formation of O-hydroxyphenylacetaldehyde from coumarin was NADPH-dependent. It was the major metabolite formed by rat, gerbil and human liver microsomes at a coumarin concentration of 1mM.