This ultrastructural study of the respiratory epithelium of the rat nasal mucosa revealed six morphologically distinct cell types: goblet cells, basal cells, ciliated cells, nonciliated columnar cells, cuboidal cells, and brush cells. The latter three have not been previously characterized in the rat nasal mucosa by transmission electron microscopy. Cuboidal cells observed on the conchae and lateral wall had short apical microvilli which were less dense than the microvilli of the nonciliated columnar cells. Nonciliated columnar cells also identified on the conchae and lateral wall had short microvilli and an extensive network of smooth endoplasmic reticulum in the apical region. The brush cell had distinct ultrastructural features; it was pear-shaped, with the broad base adjacent to the basement membrane and large microvilli on the surface. Microfilaments, microtubules, vesicles, and paired cisternae were found in the apical cytoplasm. Brush cells occurred singly on the conchae and lateral wall but were not identified in the respiratory epithelium of the nasal septum. These findings indicate the complexity of cell types composing the rat nasal respiratory epithelium.