Previous studies on the development of the parotid gland in various mammals have demonstrated that terminal clusters and acini contain mucous cells during the early postnatal period. However, little information has been available concerning the exact fate of the secretory granules in the mucous cells, specifically as to whether or not the mucous cells differentiate into serous cells. The present study aimed to determine the time of appearance of the mucous cells in the rat parotid gland as well as the morphological and histochemical changes of their granules. Light microscopy showed that cells positively stained with periodic-acid Schiff and alcian blue were sparsely distributed in the terminal clusters and acini on day 1 but had multiplied to a maximal level by day 5. They decreased in number on day 8 and were not recognizable at all by day 10. Electron microscopy revealed that the mucous cells initially contained granules of homogeneous low electron density, and then bipartite and tripartite granules with electron-dense cores were detected. By day 8 granules showing bipartite and tripartite structures and granules of homogeneous high electron density were seen to coexist in single cells. These observations suggest that mucous cells exist in parotid glands for a limited period of time and that, as the gland develops, the mucous granules come to resemble serous granules. Lectin histochemistry indicated that the secretory granules in the mucous cells were positive for peanut agglutinin, soybean agglutinin and wheat germ agglutinin, suggesting the occurrence of beta -D-galactose, alpha -D-N-acetyl galactosamine and beta -D-N-acetyl glucosamine which are the same sugar residues as those found in the granules of mature parotid serous cells. Immunostaining showed that even the low electron-dense granules in the mucous cells were weakly reactive for anti-rat parotid gland amylase; this reactivity gradually increased with development. These results suggest that mucous cell secretory granules which contain abundant glycoconjugate for a limited period during the development of the gland may change into serous granules.