Objectives: The root and stem of Sinomenium acutum has been used for treatment of arthritis and neuralgia in oriental medicine. To find new substances of the anti-anaphylactic drugs, we studied Sinomenium acutum. Methods: To investigate the effect of this plant, the effect on anaphylactic reaction, plasma histamine level, and tumor necrosis $factor-{alpha}-(TNF-{alpha})$ production were measured after the aqueous extract of Sinomenium acutum stem (SSAE) was administrated to mice and rats. Results: The SSAE (0.1 to 1000 mg/kg) dose-dependently inhibited systemic anaphylactic reaction induced by compound 48/80 in mice. Especially, SSAE reduced compound 48/80-induced anaphylactic reaction with 50% at the dose of 1000 mg/kg. SSAE (100 to 1000 mg/kg) also significantly inhibited local anaphylactic reaction activated by anti-dinitrophenyl (DNP) IgE. When mice were pretreated with SSAE at a concentration ranging from 0.1 to 1000 mg/kg, the plasma histamine levels were reduced in a dose-dependent manner. SSAE (1 to 1000 g/ml) dose-dependently inhibited histamine release from the rat peritoneal mast cells (RPMCs) activated by compound 48/80 or anti-DNP IgE. The level of cAMP in RPMCs, when SSAE was added, increased compared with that of a normal control. In addition, SSAE (0.1 g/ml) had a significant inhibitory effect on anti-DNP IgE-induced $TNF-{alpha}$ production. Conclusions: These results indicate that SSAE inhibits mast cell-mediated anaphylactic reactions and $TNF-{alpha}$ production from mast cells.