The modes of anti-inflammatory and analgesic actions of aspirin and salicylic acid were investigated using some experimental animal models. Anti-inflammatory potencies of aspirin were almost equal to those of sodium salicylate in the carrageenin hind paw edema, the cotton pellet granuloma and the adjuvant arthritis tests in rats. On the other hand, in the ultra-violet ray erythema and the arachidonic acid erythema tests in guinea pigs, aspirin was more potent than sodium salicylate. Aspirin and sodium salicylate exhibited almost the same inhibitions of the rat hind paw edema induced by a mixture of carrageenin and prostaglandin E1. These results suggest that inhibition of cyclo-oxygenase by aspirin does not play any important roles for the prevention of the vascular permeability increment and the granulation in the inflamed tissue. Aspirin may exert its antiinflammatory activity mainly as salicylic acid which is not an inhibitor of prostaglandins biosynthesis in vitro. Aspirin showed about 5 times more potent analgesic action than sodium salicylate in the lameness test using adjuvant arthritic rats. Analgesic potency of aspirin was decreased to the level of sodium salicylate by injection of prostaglandin E2 into the inflamed rat paw in the adjuvant-induced lameness test. On the other hand, analgesic potency of sodium salicylate was not decreased by the same treatment. It is concluded that aspirin has two analgesic effects on the inflammatory pain, one is inhibition of prostaglandins biosynthesis by acetylation of cyclo-oxygenase and the other is an action due to salicylic acid, but the action of salicylic acid was not totally explained by the inhibition of prostaglandins synthetase.