Abstract: Xi Xin has long been used in the treatment of arthritis in traditional Chinese medicine. "Xi Xin" essential oil was isolated from Asarum heterotropoides F. Schm. var. mandshuricum(Maxim.) Kitag. Intraperitoneal injection of "Xi Xin" oil at a dose of 0.24ml/kg reduced the swelling of the hind paw in the rat caused by injection of carrageenin. It also prevented the development of inflammation induced by carrageenin in bilaterally adrenalectomized rats. In addition, the oil decreased the histamine content of the inflammatory tissues and the exudate but, not the 5-HT and prostaglandin E₂ (PGE₂) contents. The oil reduced the swelling of the rat paw caused by injection of histamine or PGE₂ but did not reduce that caused by the injection of 5-HT. It markedly antagnized the increse of the capillary permeability due to histamine and PGE₂, while the action of 5-HT was not affected. The oil also inhibited the leucocyte migratory response caused by the injection of carrageenin and the proliferation of granuloma induced by cotton-pellet, the latter effect being accompanied by thymic atrophy and decrease of adrenal ascorbic acid content. The LD₅₀ of the oil given intraperitoneally to mice was 1.2±0.04 ml/kg.These results indicate that the mechanism of the antiphlogistic effects of the oil is rather complicated. It exerted a direct inhibitory effect on the inflammatory process in addition to a possible action through the mediation of the adrenal cortex.