The basic phospholipase A2 (PLA2) purified from Trimeresurus mucrosquamatus snake venom was injected into the subplantar in order to induce edema formation in the rat hind paw. The maximum edema induced by PLA2 was induced at 1-2 hr after injection, and the per cent swelling curve showed a dose-dependent increase by PLA2 injection (2.5-10.0 micrograms). The rate of edema formation is different from the acute swelling induced by T. mucrosquamatus venom (TMV). Pretreatment with dexamethasone (4 mg/kg, s.c.), indomethacin (10 mg/kg, per 05) and diphenhydramine (100 mg/kg s.c.) inhibited the edema induced by the purified phospholipase A2. The injection of purified PLA2 or venom into rabbit skin resulted in an increase in vascular permeability which could be decreased by pretreatment with three antiinflammatory drugs. However, the pharmacological effect of dexamethasone (4 mg/kg) demonstrated a more effective inhibition than the other drugs in the PLA2-induced edema and vascular permeability change. Injection (i.p.) of PLA2 caused marked degranulation of mast cells in the rat mesentery which was facilitated by addition of calcium ion (10 mM) but antagonized by pretreating with three antiinflammatory agents. After incubating peritoneal mast cells with PLA2 (1.0 micrograms/ml), the release of histamine from the mast cell was approximately 36%, this effect was inhibited by preincubating the mast cell with three antiinflammatory agents.