Anti-inflammatory activity of baicalein (5,6,7-trioxyflavone-7-O-beta-D-glucuronide) was greater in the chronic inflammation model (rat adjuvant arthritis, ED50 = 120.6 mg/kg) than observed in the rat carrageenan-induced paw edema, ED50>> or = 200.0 mg/kg. A comparative study of the 5-lipoxygenase (5-LO) inhibitory activity of baicalein, BW 755 C, and hydroxamic acid arachidonate on leukotriene C4 (LTC4) biosynthesis by rat resident peritoneal macrophages stimulated with calcium ionophore (A 23186) showed that these drugs significantly inhibited LTC4 production, IC50: 9.5, 41.8, and 2.8 microM, respectively. This finding suggests that inhibition of the 5-LO pathway of arachidonic acid metabolism may be one of the mechanisms of baicalein's anti-inflammatory activity.