Abstract: AIM: To investigate the effect of baicalin on leukocyte functions in attempt to elucide the mechanism of anti-inflammatory action of baicalin. METHODS: Using rat peritoneal leukocyte as experimental material to determine the levels of leukotriene B₄, C₄(LTB₄, LTC₄), Ca²⁺ and cAMP in the cells with HPLC, Fura-2/AM and RIA, respectively. RESULTS: Baicalin was shown to inhibit the biosynthesis of LTB₄ and LTC₄ with IC₅₀ value 0.48 and 3.15 μmol.L^-1, respectively. Baicalin at 1～100 μmol.L^-1 showed no effect on cytoplasmic calcium concentration (［Ca²⁺］i) of rat polymorphonuclear leukocytes (PMNL). However, baicalin at 10 and 100 μmol.L^-1 inhibited the increase of Ca²⁺ level induced by fMLP significantly by 29% and 40%, respectively. Baicalin at 1,10 and 100 μmol.L^-1 showed a significant increase of the cAMP level in rat PMNL by 74%, 103% and 138%, respectively. CONCLUSION: The effects of baicalin on leukocyte function may be the mechanisms of its anti-inflammatory action.