Abstract: Platelet activating factor (PAF)is a kind of inflammatory mediator. We used the method of aggregation of washed rabbit platelet to study the effect of esculentoside A (EsA) on the release of PAF from calcimycin (A₂₃₁₈₇) stimulated rat peritoneal macrophages and found that the release of PAF was inhibited by EsA in a time and dose dependent manner. The IC₅₀ of EsA for the inhibition of PAF release was 1.5μmol/L.Under the same condition, the release of PAF was also inhibited by mepacrine at 100 μmol/L. The results indicate that EsA is a potent inhibitor of PAF synthsis in rat peritoneal macrophages.