Tritiated thymidine was injected into young rats (age 7 days) in such a way as to be incorporated into the nuclei of all cells that were synthesizing DNA during a period of 22 hours. Specimens of the skin of the back and the pinna of the ear were taken at intervals from one to 112 days after injection of the [3H]thymidine. The mast cells were stained with Alcian blue, and autoradiographs were prepared. The nuclei were counterstained with alum-brazilin. Making due allowance for growth of the animals, and for shrinkage due to histological preparation, the total numbers of mast cells in the dorsal skin and in the pinna were determined. The numbers of mast cells containing [3H]thymidine were calculated from the proportions of those cells found to have radioactive nuclie. Using these data, the rates of appearance of labelled mast cells, and of decline in their numbers with time were determined for both regions of skin. No mitotic figures were seen in any mast cells. It is concluded that mast cells arise by the division of agranular precursor cells of unknown identity. The characteristic cytoplasmic granules appear to be produced by the daughter cells during the 24-48 hours following premitotic replication of DNA in the precursors. The differentiated cells have half-lives of 4-9 days in the skin of the back and 7-20 days in the external ear. All the labelled mast cells had disappeared after 28 days in the back and after 84 days in the ear. Comparison of these findings with the results of other investigators suggests that the mast cells produced early in life have much shorter life spans than do most of the mast cells present in adult rats, The longer life span found in the pinna may account for the greater density of the cells there than in the back. This regional difference may reflect the greater need for mast cells in a region which is more susceptible to adverse environmental influences.