The 52 alveolar brush cells (ABC) were revealed in the semi-thin sections of the rat lung tissue, metachromatically stained with toluidine blue. Characteristic features of the ABC on the light microscope level were the following: pyramidal body shape, basal position of the nucleus, darker stain tinge of the cytoplasm than that of other alveolar cells, the presence of microvilli on the small free cell surface. There is one ABC per 21 alveocytes, type 2, and 15 alveocytes, type 1. 41.1% of the ABC are localized in places of adjacent alveolus walls junction, 32.7%--on the alveolus wall facing the alveolus cavity, 16.8%--near the alveolus entrance; 9.4% of the cells are directed into the cavities of two neughbo ring alveoli or settle down near the pores of Kohn. In parallel electron microscopy there was revealed in ABC a form of granular cytoplasmic reticulum (unusual for other alveolocytes) in the form of blocks made up of 5-8 cysternae as if adherent one to another, bundles of filaments and microtubules, vacuoles in the apical cytoplasm. The ultrastructure of ABC, their topography, and incidence in the alveoli of rats evidenced their chemoreceptor nature.