Antibiotic pretreatment significantly altered metabolism of (+)-catechin (cyanidanol-3) in the rat, with decreases in both the urinary elimination of flavanol conjugates and of ring scission of the O-heterocyclic aglycone. The majority of the beta-glucuronidase(s) active in the degradation of flavanol conjugates is associated with the anaerobic microflora of the rat intestine. Ring scission is a minor reaction in vivo and appears to be due to anaerobic bacteria, although in vitro it is the major reaction and is promoted by both anaerobes and aerobes. The intestinal microflora are capable of demethylating the metabolite 3'-O-methyl-(+)-catechin in vivo; the major activity being associated with the aerobic microflora. Faecal elimination of unchanged (+)-catechin is of minor importance.