The postnatal development of the rat sublingual gland was studied using autoradiography and electron microscopy. The labeling indices of parenchymal and stromal cells were determined in autoradiographs from rats injected with ³H-thymidine. The labeling index of parenchymal cells remained high from the second to the twentieth day after birth, but declined significantly thereafter. Ultrastructural analysis showed that during the period of high proliferative activity the secretory cells had an ultrastructure that was qualitatively similar to that of cells from the glands of old rats, but were still immature. Only after day 20 did these cells show large amounts of rough endoplasmic reticulum and secretory granules that reached the adult pattern 30 days after birth. Maturation of the myoepithelial cells occurred during the same period.