Comparative studies on the parafollicular cells of the some mammalian species from five different orders were carried out; i.e., man from Primates, cattle, pig, and black goat from Artiodactyla, dog from Carnivora, rabbit from Lagomorpha, rat, mouse, and squirrel from Rodentia. For this study, various special techniques for the parafollicular cells, including Grimelius' silver impregnation method (Sawicki and Bajko, 1974), Singh's argentaffin method (Singh, 1964), HCl-toluidine blue stain (Sawicki, 1971), and HCl-lead hematoxylin stain (Solcia et al., 1969), were applied. Authors obtained the following results: 1. Number of parafollicular cells in the same area of thyroid tissues are significantly different from species to species. Number of cells were largest in dog and less cells were found in the following orders; rat, squirrel, mouse, rabbit, cattle, pig, black goat and finally the smallest number in man. 2. Distribution of parafollicular cells within thyroid gland are significantly different from portion to portion in case of cattle, rabbit, squirrel and mouse, but it is not significant in dog, man, pig, black goat and rat (see Table 1-1 and 1-2). 3. In dog, clustered parafollicular cells are located usually in the interfollicular space, and groups of parafollicular cells are located in the para-and/or inter-follicular positions in rabbit. But in the other animals parafollicular cells are found solitarily in the intra-and/or para-follicular positions. 4. The shape of parafollicular cells shows oval to round contour in dog, but it is polymorphic, for example, spindle, conical, oval, round or elongated with cytoplasmic processes, in the other animals. 5. Size of parafollicular cells is larger in cattle, dog and pig, smaller in rat, mouse and squirrel, and medium-size in rabbit, man and black goat. 6. Parafollicular cells of pig, cattle, dog and squirrel are observed to contain densely packed granules, whereas those of mouse, rat and man contain relatively scanty granules. 7. Parafollicular cells of all the mammals show more or less positive reaction to Grimelius' argyrophile silver impregnation method, HCl-toluidine blue stain and HCl-lead hematoxylin stain, whereas they show negative reaction to argentaffin method (see Table 2). 8. Considering the above finding, it is concluded that there are species differences in the distribution, location and shape of parafollicular cells, and infer that preferable staining method should be selected for reliable detection of parafollicular cells, beacuse staining methods applied on the cells in this study show variable reactions according to species.