Fifteen days after thyroidectomy, the pituitary acidophils of rats were definitely deprived of their large granules, about 350mμ in diameter each, and in due succession, they displayed the changes characterized by the features of “thyroidectomy cells” (TXcells). When these rats were given a single or three injections of thyroxine, 50mg/rat, most of TX-cells might revert to their original acidophils in a short time of 6 hr. The cytomorphological course of possible reversion of the TX-cells into acidophils was clearly demonstrated on our electron micrographs. It was learned that thyroxine exhibited a strong ability to produce large granules, 350mμ in diameter each, in TX-cells which might return to their original acidophils through the reduction of cell size probably due to contraction of ER and loss of intracisternal granules and through the re-production of the large granules in the ground matrix. If the rats, 15 days after thyroidectomy, were given dexamethasone (DM), 0.2mg/100g/day, for 14 days, most of TX-cells tended to slightly recover from their modifications due to thyroidectomy. DM had ability to produce both the small (130-150mμ in diameter) and the large (350mμ) granules in some TX-cells, but the rest of them remained unchanged and preserved a number of dilated cisternae and intracisternal granules. DM was greatly inferior to thyroxine in the ability to produce the acidophils of the large granule type (somatotrophs), while the former was superior to the latter in the ability to produce the acidophils of the small granule type. Our previous views that some TX-cells might originate from the degranulated acidophils was confirmed validly by the present observation. In conclusion, all TX-cells have not always been considered as hyperactive thyrotrophs. Mechanism of ER morphodynamics associated with vesiculation in TX-cells has been discussed with special reference to the development and reversion of TX-cells.