The activity of endogenous peroxidase was demonstrated in light ciliated cells, mature goblet cells, and nonciliated serous cells, but not in basal cells, intermediate cells, dark ciliated cells or immature goblet cells of the upper tracheal epithelium of rats, using the diaminobenzidine method for cytochemical demonstration of peroxidase activity. The peroxidase activity ws the most intense in the epithelium of the first tracheal ring and gradually diminished at lower tracheal levels, becoming extremely faint in the vicinity of the middle of the trachea. The activity was localized in cisternae of rough-surfaced endoplasmic reticulum (r-ER) including the nuclear envelope, some vesicles and saccules of the Golgi complex, secretory granules, and small apical vesicles. It is believed that the basal cells were transformed into the intermediate cells, and some of the intermediate cells were turned into the ciliated cells, while other intermediate cells were differentiated into the goblet cells. The dark ciliated cells proved to be the immature ciliated cells immediately after the transformation from the intermediate cells. In these cells, peroxidase is not yet produced.