The antiallergic properties of the 70% ethanol extract from Plumbago zeylanica stems (EPZ) were investigatedin the present study. The extract (500, 1000mg/kg, p.o.) dose-dependently inhibited systemic anaphylacticshock induced by compound 48/80 in mice, reduced homologous passive cutaneous anaphylaxis and skin reactionsinduced by histamine or serotonin in rats, significant differences were observed at the dose of 1000 mg/kg.In vitro, EPZ (5, 20, 50mg/ml) concentration-dependently reduced histamine release from rat peritoneal mastcells caused by compound 48/80 and antigen. EPZ (50mg/ml) markedly increased intracellular cAMP content ofrat mast cells. These findings demonstrate that EPZ inhibits mast cell-dependent immediate allergic reactions,which is probably mediated by reducing the release of mediators such as histamine from mast cells via elevatingintracellular cAMP level and weakening the inflammatory action of mediators.