Chlorogenic acid, the ester of caffeic acid with quinic acid, is one of the most abundant polyphenols in the human diet with coffee, fruits and vegetables as its major sources. Its antioxidant and anticarcinogenic properties have been well established in animal studies. However, little is known about its gut absorption and metabolism. In the present work, four groups of rats (n = 8) were fed a diet supplemented with chlorogenic, caffeic or quinic acids (250 micromol/d) or an unsupplemented diet for 8 d. Parent compounds and their metabolites were estimated in urine (24-h collection) and plasma by HPLC-electrospray ionization-tandem mass spectrometry. Significant differences in their levels were observed among the groups. The recovery of chlorogenic acid in urine was low (0.8%, mol/mol), and the total urinary excretion of caffeic acid liberated by hydrolysis of chlorogenic acid and its tissular methylated metabolites (ferulic and isoferulic acids) did not account for >0.5% (mol/mol) of the dose ingested. On the other hand, the metabolites of microbial origin, namely, m-coumaric acid and derivatives of phenylpropionic, benzoic and hippuric acids, represented the major compounds in both urine and plasma. Hippuric acid largely originated from the transformation of the quinic acid moiety, and all other metabolites from the caffeic acid moiety. These microbial metabolites accounted for 57.4% (mol/mol) of the chlorogenic acid intake. Such a high abundance of microbial metabolites shows that the bioavailability of chlorogenic acid depends largely on its metabolism by the gut microflora. Their potential importance in explaining the biological effects of dietary polyphenols is emphasized.