1. Following oral administration of [U-14C]-(+)-catechin to the rat, the major biliary metabolite was shown to be the glucuronide of 3'-O-methyl-(+) catechin by chromatography and mass spectrometry. 2. [methyl-14C]-O-Methyl-(+)-catechin was formed by incubation of (+)-catechin with S-adenosyl-L-[methyl-14C]methionine in vitro in both liver homogenates and in the presence of purified catechol-O-methyl transferase. 3. Alkaline micro-fusion techniques have been used to determine the position of the O-methyl substituent in the B-ring of the isolated O-methyl-(+)-catechin glucuronide.