An electron microscopic study of the exocrine and myoepithelial cells of the parotid gland of the rat between days 2 and 40 of the postpartum was undertaken. Besides confirming morphological data of the literature on the evolution of the acinar cells, the following observations are reported. 1. At early postnatal development in some acini "mucous-like" cells containing secretory granules with light and dense parts may be observed. Cells with these types of granules were not found after day 20. 2. Morphological differentiation and maturation of the myoepithelial cells were followed. A parallelism was observed between the maturation of these myoepithelial cells and of the acinar cells. Myofilaments first appear around the nucleus at day 5 and between days 20 and 30 a considerable amount of these filaments is found in the already typical myoepithelial cells. 3. Active morphogenesis seems to be occurring in all cellular compartments namely at very early postnatal periods, e.g., between days 2 to 15. 4. The distribution and amount of thin and intermediate sized microfilaments in cells of these compartments varied visibly by various orders of magnitude.