The absorption and metabolism in the small intestine of chlorogenic acid (5-O-caffeoylquinic acid), the main phenolic acid in the human diet, and of caffeic acid were studied in rats in order to determine whether chlorogenic acid is directly absorbed or hydrolysed in the small intestine. Chlorogenic and caffeic acids were perfused into a segment of ileum plus jejunum during 45 min (50 microm, 0.75 ml/min) using an in situ intestinal perfusion rat model with cannulation of the biliary duct, and were quantified together with their metabolites in perfusion effluent, bile and plasma. The net absorption (influent flux minus effluent flux of phenolic acids and their metabolites) accounted for 19.5 % and 8 % of the perfused caffeic and chlorogenic acids, respectively. A minor fraction of the perfused caffeic acid was metabolized in the intestinal wall and secreted back into the gut lumen in the form of ferulic acid (0.5 % of the perfused flux). Part of the chlorogenic acid (1.2 % of the perfused flux) was recovered in the gut effluent as caffeic acid, showing the presence of trace esterase activity in the gut mucosa. No chlorogenic acid was detected in either plasma or bile, and only low amounts of phenolic acids (less than 0.4 %) were secreted in the bile. The present results show that chlorogenic acid is absorbed and hydrolysed in the small intestine. In contrast to numerous flavonoids, absorbed phenolic acids are poorly excreted in the bile or gut lumen. Their bioavailability therefore appears to be governed largely by their uptake into the gut mucosa.