The fine structure of the acute allergic reaction is described in the human nasal mucosa. Subsequent to intranasal allergic testing with positive reaction characteristic changes are seen after different time intervals: Massive transudation into the dilated intercellular spaces, loss of epithelial junctions, extrusion of epithelial cells into the nasal lumen; increase permeability of capillaries and venules with increase of fenestrations and pinocytotic vesicles, edema of the cytoplasm and dilatation of intercellular junctions in the endothelium. Characteristic for the later stages are pronounced lesions of the vessel walls with interruption of cell junctions interstitial edema. endothelial gaps and loss of endothelial cells. The endothelial basal membrane usually remains intact. The findings are compared with those described for the acute hyperergic reaction in animals and chronic allergic changes in the human nasal mucosa.