Antagonistic activities of Mequitazine (LM-209) on chemical mediators and in particular histamine were investigated in guinea-pigs, mice and rats. The antagonistic activity of LM-209 for histamine in the isolated ileum and trachea of guinea-pigs was less potent than that for clemastine fumarate (CL) and chlorpheniramine maleate (CPM) while that for acetylcholine was more potent than CL and CPM. Moreover, the antagonistic activities of these three compounds on serotonin and bradykinin were almost equipotent in the excised ileum. Using a modified Konzett-Rössler method, the bronchodilating effect of LM-209 (p.o.) for histamine was same as CL, but that for acetylcholine was more potent than CL and CPM. The protective activity of LM-209 (p.o.) on acute death induced by histamine and metacholine in mice was the same as CL, but the duration of the anti-histaminic action was markedly longer than CL. LM-209 given orally inhibited markedly the increased vascular permeability by histamine in rats and the diarrhea by 5-HTP in mice, but did not affect on the histamine-induced ulcer in guinea-pigs. From these results, LM-209 appears to have potent and long acting antagonistic activity on various chemical mediators.