The aim of this research was to characterize the population of thymic mast cells (MC) which were demonstrated by histochemical methods, at different stages of mouse ontogenesis. First MCs appeared on day 19 of intrauterine life. MCs were localized to the thymic medulla during the whole embryonic period. Individual MCs could also be identified within the cortex. Single MCs were interspersed among other thymic medullary cells. In the newborn, young and mature animals, MCs were observed in the connective tissue of the capsule and in the interlobular septa, but some MCs were also found inside the thymic lobules. In the last-mentioned case, MCs were located in the subcapsular zone. The possible functional significance of MCs localization is discussed.