Abstract: The present study is aimed to investigate the in vitro metabolic interconversion between baicalin (BG) and baicalein (B) in rat liver, kidney, intestine and bladder. BG and B were separately incubated with rat hepatic, renal, and intestinal microsomes, as well as bladder homogenates, for 30 min. The metabolites were identified and quantified by HPLC and metabolic kinetic parameters were obtained by fitting the data to the Michaelis-Menten equation. In hepatic microsomes, renal microsomes and bladder homogenates, but not in intestinal microsomes, BG was transformed into B, the hydrolysis metabolite of BG, with K_m values being (44.65±6.01), (92.73±11.41), (74.60±3.68) μmol·L^-1, respectively, and V_max values being (12.32±0.56), (3.30±0.18), (5.93±0.12) μmol·min^-1·g^-1 (protein), respectively. In incubations with hepatic, renal, and intestinal microsomes and bladder homogenates, B was also transformed into BG, the glucuronidation metabolite of B, with K_m values being (67.46±10.49), (226.7±71.59), (177.3±35.85), and (18.33±2.53) μmol·L^-1, respectively, and V_max values being (14.74±0.97), (5.91±1.03), (38.14±3.60), and (1.22±0.05) μmol·min^-1·g^-1 (protein), respectively. The results showed that the activity of UDP-glucuronosyltranferase (UGT) in intestinal microsomes was the highest among the four organs, and the activities of UGT were higher than that of glucuronidase (GUS) in hepatic, renal and intestinal microsomes, but the activity of GUS was higher than that of UGT in bladder homogenates.