Objective To study the effects of natural platelet activating factor(PAF) receptor antagonist (kadsurenin mixture and ginkgolide B)on cyclooxygenase 1 (COX 1) and cyclooxygenase 2 (COX 2) messenger RNA (mRNA) expression in rat brain tissue during ischemia reperfusion Methods Fourty elderly Wistar rats were divided into eight groups: sham operated control group, simple ischemia for 90 min group, 6 h , 12 h , 24 h , 48 h reperfusion groups, kadsurenin mixture treated and ginkgolide treated groups After right middle cerebral artery occluded, rats underwent ischemia for 90 minutes, then reperfusion was allowed for different time as mentioned above before decapitation Reverse transcription polymerase chain reaction (RT PCR) was applied to evaluate the COX 1 and COX 2 mRNA expression levels in rat brain tissue of ischemia reperfusion Results RT PCR analysis revealed that the levels of mRNA of COX 2 of 6 h , 12 h , 24 h and 48h reperfusion groups were all significantly higher than the sham operated control group's(density ratio:0 27±0 05). The climax of the levels of COX 2 mRNA was at 12 h(1 22±0 19) Both PAF receptor antagonist inhibited the increased expression of COX 1 mRNA(0 58±0 11 vs 0 68±0 13)in 12 h reperfusion group significantly Conclusions The cerebral ischemia reperfusion can induce the enhancement of COX 2 mRNA expression PAF is an upper stram signal in the expression of COX 2 mRNA Reducing the expression of COX 2 mRNA might be one of the neuroprotective machnism of two natural PAF receptor antagonists