It has previously been known that the developing parotid glands in humans and rats contain mucous cells in their terminal clusters and acini, but these cells disappear within a short period of time. Using rat parotid glands, IKEDA and AIYAMA (1997, 1999) suggested that the mucous cells might change into serous cells in the early postnatal period, but it is uncertain whether mucous cells appear only in the developing parotid gland of a few species such as humans and rats, or whether the cell transformation actually occurs. To clarify these points, the present study investigated the developing mouse parotid glands. Light microscopy showed cells with secretory granules that stained extensively with PAS and alcian blue in the terminal clusters of a 1-day-old mouse parotid gland. Mucous cell numbers in the terminal clusters and the acini reached a peak on day 5 and decreased on day 7. By day 10, the mucous cells had disappeared altogether. Thus, the presence of mucous cells in the developing mouse parotid gland was confirmed. Electron microscopy showed granules of low-electron-density and bipartite granules in the mucous cells. Bipartite granules and highly electron-dense granules sometimes co-existed in a single cell. Immuno-electron microscopy revealed a positive reaction for amylase to the low-electron-density granules and the low-electron-density portions of the bipartite granules, in addition to the highly electron-dense granules and the electrondense cores of the bipartite granules. No mucous cells with nuclei displaying characteristics of apoptosis were recognizable. Lectin histochemistry both at the light and electron microscopic levels showed that the secretory granules in the mouse parotid gland mucous cells had sugar residues similar to those of the mature serous granules. These findings demonstrate that mucous cells appear in the early postnatal mouse parotid gland, and that almost all of these cells may be converted into serous cells.