The purpose of this study was to clarify histochemically the relation of ultrastructure and function in osteoclasts treated with calcitonin during experimental tooth movement. The upper first molars of Wistar strain rats were moved with a wire spring to obtain a number of active osteoclasts. These rats were divided into a control group (not treated with calcitonin) and seven experimental groups (treated with calcitonin). For demonstration of cytochrome c oxidase activity in mitochondria of the osteoclasts, the DAB method of Seligman et al. was used. The following results were obtained. 1) There was no remarkable difference in the localization of the enzyme activity of osteoclasts between undecalcified tissues and tissues decalcified by 5.0% EDTA solution. 2) Active osteoclasts in the control group contained many mitochondria with reaction products of the enzyme in the inner membrane and outer compartment (type 1 mitochondria, 82.6%/100μm² in an osteoclast except nuclei) and a few mitochondria with reaction products only in the inner membrane of the cristae including intracristal spaces or lacking them at all sites (type 2 mitochondria, 17.4%). 3) At 3hr after calcitonin administration, maximal morphological changes of the osteoclasts with disappearance of a ruffled border and striking decrease of type 1 mitochondria (12.2%) were observed. 4) Osteoclasts of the experimental group at 72hr after calcitonin administration were almost typical active osteoclasts as seen in the control group and contained 80.2% type 1 mitochondria. These results suggest that cytochrome c oxidase activity was closely associated with changes in form and function of the osteoclasts.