To clarify the metabolic fate of glycyrrhizin when orally ingested, we investigated the bioavailability of glycyrrhetic acid, the aglycone of glycyrrhizin, after intravenous or oral administration of glycyrrhetic acid (5.7 mg kg-1, equimolar to glycyrrhizin) or glycyrrhizin (10 mg kg-1) at a therapeutic dose in rat. Plasma concentration of glycyrrhetic acid rapidly decreased after its intravenous administration, with AUC of 9200 +/- 1050 ng h mL-1 and MRT of 1.1 +/- 0.2 h. The AUC and MRT values after oral administration were 10600 +/- 1090 ng h mL-1 and 9.3 +/- 0.6 h, respectively. After oral administration of glycyrrhizin, the parent compound was not detectable in plasma at any time, but glycyrrhetic acid was detected at a considerable concentration with AUC of 11700 +/- 1580 ng h mL-1 and MRT of 19.9 +/- 1.3 h, while glycyrrhetic acid was not detected in plasma of germ-free rats at 12 h after oral administration of glycyrrhizin. The AUC value of glycyrrhetic acid after oral administration of glycyrrhizin was comparable with those after intravenous and oral administration of glycyrrhetic acid, indicating a complete biotransformation of glycyrrhizin to glycyrrhetic acid by intestinal bacteria and a complete absorption of the resulting glycyrrhetic acid from intestine. Plasma glycyrrhizin rapidly decreased and disappeared in 2 h after intravenous administration. AUC and MRT values were 2410 +/- 125 micrograms min mL-1 and 29.8 +/- 0.5 min, respectively. Plasma concentration of glycyrrhetic acid showed two peaks a small peak at 30 min and a large peak at 11.4 h, after intravenous administration of glycyrrhizin, with an AUC of 15400 +/- 2620 ng h L-1 and an MRT of 18.8 +/- 1.0 h. The plasma concentration profile of the latter large peak was similar to that of glycyrrhetic acid after oral administration of glycyrrhizin, which slowly appeared and declined. The difference of MRT values (19.9 and 9.3 h) for plasma glycyrrhetic acid after oral administration of glycyrrhizin and glycyrrhetic acid suggests the slow conversion of glycyrrhizin into glycyrrhetic acid in the intestine.