The formation and morphological maturation of Bergmann glia cells, which might play an important role in the histogenesis of the cerebellar cortex, was studied by histological and electron microscopic techniques in the cerebellum of normal young rats and animals made hypothyroid since the 18th day of gestation or hyperthyroid since birth. In comparison with controls, an acceleration in the morphological development of Bergmann glia cells was observed in the hyperthyroid animals; on the contrary, in the thyroid-deficient rats, the formation and morphological maturation of these cells were retarded and their final number was increased.