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Tioxamast (F 1865) is an antiallergic drug that, administered systemically, reduces anaphylaxis in various models in rats. This action is due mainly to the inhibition ofthe synthesis and release of certain mediators. Orally or intraduodenally administered tioxamast inhibits IgE-dependent passive cutaneous anaphylaxis (ED50 = 0.8 mg/kg), IgE-dependent passive pulmonary anaphylaxis (ED50 = 0.5 mg/kg), and IgG-dependent passive cutaneous anaphylaxis (ED50 = 0.6 mg/kg). It has little or no effect on the increase of cutaneous capillary permeability induced by various mediators. In IgE-dependent passive peritoneal anaphylaxis in rats, tioxamast reduces the release of histamine (IC50 = 0.024 µg/ml) and of β-glucuronidase (IC50 = 0.102 µg/ml). Also, histamine release is inhibited in IgG-dependent peritoneal anaphylaxis (IC50 = 0.103 µg/ml). The antiallergic compound has less effect on the release of histamine induced by the compound 48/80 in the peritoneal cavity of rats (IC50 = 1.67 µg/ml). Tioxamast inhibits the synthesis in vitro of leukotriene B4 (LTB4) by peritoneal neutrophils from rats stimulated by A23187 (IC50 = 8.88 µg/ml). At higher tioxamast concentrations, metabolites ofthe cyclo-oxygenase pathway are inhibited at concentrations ofthe same order of magnitude as those that inhibit Naja naja phospholipase A2 (IC50 = 144 µg/ml). Tioxamast also reduces the production of free radicals by leukocytes from the pleural cavity of rats which had phagocytosed opsonized zymosan (IC50 = 5.21 µg/ml).