The mechanism of gastrointestinal absorption of glycyrrhizin (GZ) was examined in rats. Using an in situ loop technique, it was found that the intestinal absorption of glycyrrhetic acid (GA), a major metabolite of GZ, was larger than that of GZ and that the absorption of GA was larger in the small intestine than in the large intestine. Although GZ was poorly absorbable, both GZ and GA were detected in rat plasma after oral administration of GZ, suggesting that GZ can be absorbed in both parent and metabolite forms, although their bioavailabilities were low. GZ was hydrolyzed to GA by rat gastric and large-intestinal contents, but not by the small-intestinal contents. This hydrolysis was diminished after boiling of the gastrointestinal contents and was not observed in the gastrointestinal contents of kanamycin-treated rats. These results suggest that GZ is hydrolyzed by bacteria in the stomach and large-intestinal contents and that most of the GA formed is absorbed from the large intestine. Since GZ was extensively excreted in bile after intravenous administration, the first-pass elimination might be the reason for its low bioavailability, in addition to the poor mucosal permeability.