Effects of the aqueous extract of Aquilariae Lignum (Thymelaeaceae) on the allergic reactions were investigated. Oral administration of this extract (50, 250, and 500mg/kg) exhi bited a dose-dependent inhibition on passive cutaneous anaphylactic reactions in rats. Administrations of this extract (500mg/kg, i.p.) at 60 min before and 5, 10 min after the compound 48/80 treatment (8mg/kg, i.p.) decreased the mortality rates to 0, 0, and 14.2%, respectively. The aqueous extract of Aquilariae Lignum (0.05 ~ 1.6mg/ml) showed a dose-related inhibition on histamine release from rat peritoneal mast cells. The morphological examination also clearly showed that the aqueous extract of Aquilariae Lignum prevented the degranulation of mast cells in rats.