The effect of aqueous extract of Lycopus lucidus Turcz. (Labiatae)(LLAE) on mast cell-mediated immediate-type allergic reactions was investigated. LLAE (0.01 to 1 mg/g) dose-dependently inhibited systemic anaphylaxis induced by compound 48/80. LLAE (0.001 to 1 mg/g) also dose-dependently inhibited local anaphylaxis activated by anti-dinitrophenyl (DNP) IgE. When LLAE was pretreated at the same concentration with systemic anaphylaxis, serum histamine levels were reduced in a dose-dependent manner. LLAE (0.001 to 1 mg/mι) dose-dependently inhibited histamine release from rat peritoneal mast cells (RPMC) activated by compound 48/80. The level of cAMP in human mast cell line (HMC-1) cells, when LLAE (1 mg/mι) was added, significantly was increased, compared with that of normal control. These results provide evidences that LLAE may be beneficial in the treatment of allergic diseases.