The health effects of dietary polyphenols might be explained by both intact compounds and their metabolites formed either in the tissues or in the colon by the microflora. The quantitative importance and biological activities of the microbial metabolites have seldom been examined in vivo. We measured the microbial metabolites formed in four groups of rats (n = 8) fed for 8 d a diet supplemented with 0.12 g/100 g catechin, 0.25 or 0.50 g/100 g red wine powder containing proanthocyanidins, phenolic acids, flavanols, anthocyanins and flavonols or an unsupplemented diet. Fourteen aromatic acid metabolites were assayed in urine collected for 24 h by an HPLC-electrospray ionization (ESI)-mass spectrometry (MS)-MS method. The three main metabolites formed from the catechin diet were 3-hydroxyphenylpropionic acid, 3-hydroxybenzoic acid and 3-hydroxyhippuric acid. Their total urinary excretion accounted for 4.7 g/100 g of the catechin ingested and that of intact catechins for 45.3 g/100 g. For wine polyphenols, the same microbial metabolites as observed for the catechin diet were identified in urine along with hippuric, p-coumaric, vanillic, 4-hydroxybenzoic and 3-hydroxyphenylacetic acids. All together, these aromatic acids accounted for 9.2 g/100 g of the total wine polyphenols ingested and intact catechins for only 1.2 g/100 g. The higher excretion of aromatic acids by rats fed wine polyphenols is likely due to their poor absorption in the proximal part of the gut. Some of the microbial metabolites still bear a reducing phenolic group and should also prevent oxidative stress in inner tissues. More attention should be given in the future to these microbial metabolites and their biological properties to help explain the health effects of polyphenols that are not easily absorbed through the gut barrier.