The thyroid glands of 3 year old rats were studied by means of electron microscope. The epithelial activity and the maintenance of tissue very markedly in different parts of the organ. The most thyreocytes are characterized by hypofunction. The rough endoplasmic reticulum is poorly developed, the formation of secretory vesicles is decreased, the signs of colloid resorption from the follicular cavity are absent, and many cells are overloaded with pigment-lipid granules. Some thyreocytes have intranuclear inclusions and others have polyploid nuclei. As a rule, mitochondria are swollen and have light matrix and disconnected cristae. Cells with large branchy mitochondria having dense matrix and tubular cristae were found which may be classed as a particular state of thyreocytes. Psammous bodies and desquamated epithelium were observed in some follicles. The amyloid-like material and large bundles of collagen fibrils appear among the follicles and capillaries and are, sometimes, in direct contact with the thyreocytes. The condensation and spreading of stromal elements contribute to the development of hypothyroidism.