The antiallergic activity of rhubarb and its constituents, anthraquinones, has been reported previously. For further evaluation of the antiallergic activity, a 70% ethanol extract of the rhizomes of Rheum tanguticum (RTE) was prepared and its inhibitory activity on an animal model of atopic dermatitis (AD) was examined for the first time. Oral administration of RTE (30-300 mg/kg/day) for 5 weeks significantly inhibited hapten-induced dermatitis in NC/Nga mice based on the skin severity score. In addition, treatment with RTE at 100 mg/kg/day also reduced the numbers of white blood cells, neutrophils and eosinophils in the blood, and led to a significant reduction in the IgE concentration in the serum. In rat basophilic leukemia (RBL)-1 cells, RTE inhibited 5-lipoxygenase (5-LOX)-catalysed leukotriene production (IC(50) = 43.6 µg/mL). Among the anthraquinone derivatives isolated, emodin strongly inhibited this parameter (IC(50) = 4.3 µM). Taken together, these findings suggest that rhubarb exerts inhibitory activity against AD, and that the 5-LOX inhibitory activity of its major constituent, emodin, may contribute to this inhibitory action.