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In order to ascertain the mode of anti-inflammatory action of a topical non-steroidal anti-inflammatory drug, etofenamate which is a diethylene glycol ester of flufenamic acid, the in vitro test for the mechanism of the action were carried out. Etofenamate (3 microM) was hydrolysed to flufenamic acid at a rate of 39.5% and 57.0% of the dose during 30 and 60 min incubation, respectively, when incubated with rat peritoneal macrophages stimulated with starch and bacto peptone in phosphate-buffered saline. PGE2 generation by these cells in MEM medium was dose-relatedly inhibited with etofenamate as well as flufenamic acid at the dosage range of 1 to 30 microM. This suggests that unchanged etofenamate is active, since the highest conversion rate of etofenamate to flufenamic acid was 15% of the dose during the incubation. Etofenamate produced a dose-related inhibition against lipoxygenase prepared from peritoneal polymorphonuclear leucocytes of guinea pigs, and its activity (IC50 = 5.3 X 10(-5) M) was stronger than that of caffeic acid; flufenamic acid was inactive. Inhibitory activity of etofenamate was one-third or less that of flufenamic acid against the hypotonic-hyperthermic lysis of rat erythrocytes and heat-denaturation of bovine serum albumin. From these results, it was suggested that topically applied etofenamate produces its anti-inflammatory action through prostaglandin synthesis inhibition by flufenamic acid produced in the inflammatory tissue and inhibition of prostaglandin synthesis by macrophages and lipoxygenase inhibition by unchanged etofenamate.