With the materials of adult male rats, mice, rabbits, guinea pigs, golden hamsters, dogs, cats, oxen, castrated horses, castrated or non-castrated pigs, BASEDOWian women (aging 35, 35, 38) and human embryos of 4 months, the electron microscopic observations were undertaken to enumerate the basic criteria of cellular ultra-fine structures of thyroid glands in reference to the comparative examination of various mammals and the results obtained are as follows:1. The endothelial cells of sinusoids abundantly existing between the follicles provide the splited spaces (pores), approxymately 400Å in width, which are covered with the thin plates 50Å in thickness. These cytoplasmic discontinuations are discernible in thyroids of all mammals, and their frequency is same with the various animals. Both luminal and basal sides of the endothelial cells are invaded by the irregular but short infoldings of the cell membrane whose development is also negligible of the animal-differences.2. The basal membrane of the follicular epithelium consists of three layers: The central one is of lower density than two outer sheets. It was frequently encountered in mice and oxen that the former looks too equivocal to be interposed between the latters.3. So-called basal infoldings in the follicular epithelial cells usually perpendicularly, diagonally, sometimes sinuously in a non-straight line, enter the cell-bodies from the basal parts, but never invade the innermost region. A part of cytoplasm confined by them often shows a islet-like appearance; the extension of infoldings is the most complicated in hamster, followed by the BASEDOWian women and mouse, and in the other animals they are poorly developed.4. The large follicles have the low epithelium whose composing cells are also squamous in profile. The cytoplasm contains in general no expanding endoplasmic reticulum (ER), but a number of tiny closing one, a few mitochondria (Mt) and majorities of microsomes respectively. These findings in low epithelial cells exhibit a striking contrast to the ultra-fine structures in the high columunar epithelial cells composing the wall of small follicles; the case is reversible in distribution and shapes of ER, Mt and microsome. However the cell height index of the BASEDOWian thyroid gland has not always a close correlation to the size of the follicle, since the two types of cells, pale or dark as well as its intermediary cell, are concurrently recognized; it was substantiated that the pale cell is characterized by contained abundant ER, whereas the dark cell is done by numerous microsomes. The identification of two cells is facilitated in rats, rabbits, oxen and horses.5. The nuclei of the follicular cells are provided with the double membrane which are often penetrated by the pores 400Å in diameter. Nuclear membrane locally falls deeply into the nuclear ground substance, followed by an insertion of infoldings connected with the ER. These infoldings or incisurae of the nuclear membrane are found at least in 1-3 places on a cross-section of the nucleus without an exception throughout the animals. The nucleolus is as a rule situated in the nuclei of all animals with extraordinary frequency in the human embryos.6. The ER appearing within the follicular cells is in general irregular, polygonal and spherical in shape, and swollen or expanding type in spite of seldom occurrence of the lamellar type. In particular the development of ER is remarkable in rats, and in decreasing sequence are BASEDOWian women, oxen, horses, dogs, cats. Expanding ER in above animals is principally accumulated at the supranuclear regions with a close connection to the mitochondria in such manner that ER invaginates the spherical, ellipsoid, short and long rod-shaped mitochondria. The latter is swollen at the ends enclosed by the smooth surfaced ER, where the cristae mitochondriales completely disappear.