The postnatal development of rat sublingual glands was analyzed by morphometric and radioautographic studies. The absolute number of each cell type was evaluated by the Aherne II morphometric method for cell counting and labeling indices of these cell types were determined in radioautographs from animals injected with 3H-thymidine. The quantitative cell population kinetic studies were accompanied by morphologic analysis of the modifications in each gland structure. The data concerning evolution of number of each cell type were submitted to analysis by least squares fit-exponential curve. The exponential equations duplication times for the acinar, serous demilune, intercalated duct, striated duct and stroma cells from 2 to 30 days of age were 7.5, 9.0, 10.8 and 9.5 days, respectively. On the other hand, the mean labeling indices for the same cell types during the same period were 9.5%, 5.8%, 7.2%, 3.3% and 4.3%, respectively. Thus, the intercalated duct cells exhibited the second highest labeling index and the slowest growth rate, while the striated duct cells showed the lowest labeling index and the third highest duplication time. The fact that the striated duct cell labeling index does not explain the relatively short duplication time of these cells, suggests that cells from other neighboring morphologic compartments, probably from intercalated duct, migrate and differentiate into striated ducts cells.