Histological, electron-microscopic and morphometric investigations of the thyroid gland of Wistar SPF male rats (aged 3 months) flown for 7 days on Cosmos-1667 showed that its parenchyma was functionally active and changed but little as compared to the controls. However, at an acute stage of adaptation to microgravity C-cells showed morphological signs of their functional decline: the number of low activity cells and cells whose cytoplasm contained secretory granules increased, the volume of nuclei decreased significantly (by 16.2% as compared to the control), and dystrophic changes seen ultrastructurally appeared. These observations together with the results obtained in prolonged animal flights suggest that in microgravity the synthesis and excretion of the hormone calcitonin diminish. In combination with other factors, the functional decline of C-cells inhibits bone neoformation and enhances bone resorption.