Tea is a popular beverage consumed worldwide. The metabolic fate of its major constituents, catechins, however, is not well-known. In this study, two catechin metabolites were detected in the urine and plasma of human volunteers after ingestion of green tea. These metabolites were identified by LC/ESI-MS and NMR as (-)-5-(3',4', 5'-trihydroxyphenyl)-gamma-valerolactone (M4) and (-)-5-(3', 4'-dihydroxyphenyl)-gamma-valerolactone (M6). The renal excretion of M4 and M6 had a 3 h lag time and peaked 7.5-13.5 h after ingestion of a single dose of green tea, while (-)-epigallocatechin (EGC) and (-)-epicatechin peaked at 2 h. M4 and M6 were two major tea metabolites with urinary cumulative excretions as high as 8-25 times the levels of EGC and (-)-epicatechin in some of our subjects, and accounted for 6-39% of the amounts of ingested EGC and (-)-epicatechin. Both the metabolites appeared to be produced by intestinal microorganisms, with EGC and (-)-epicatechin as the precursors of M4 and M6, respectively. Repeated ingestion of green tea produced a slight accumulative effect of the metabolites. They were also detected in the plasma, exhibiting kinetics similar to those of the urinary metabolites, and in the feces. Study on these metabolites may help us further understand the cancer chemopreventive actions and other beneficial effects of tea.