The kinetics of mast cell accumulation in the nasal mucosa during allergen exposure were investigated in animal model of experimentally induced allergic rhinitis. Guinea-pigs were divided into an unsensitized control group and five sensitized groups each containing live animals. Five sensitized groups were immunized intraperitoneally with ovalbumin, followed by intranasal administration of ovalbumin in each of four groups for 1. 2, 3 and 4 weeks, respectively. There were significant increases in the number of mast cells in the epithelium after 2 and 3 weeks in the groups receiving intranasal ovalbumin. No significant changes were detected in the lamina propria and the total number of mast cells. In the lamina propria, the distance between a mast cell and the basement membrane was significantly decreased in the groups after 2, 3 and 4 weeks of intranasal administration. These findings suggest that an infiltration of mast cells from the lamina propria to the epithelium occurred after 2 weeks of intranasal administration of allergen. An electron microscopic study showed no mast cells at the luminal surface of the epithelium and the majority of mast cells lying around the basal cells. This finding suggests that the allergen must penetrate into the epithelium in order to interact with mast cells in guinea-pig nose.