SYNOPSIS Healthy adult male rats were given test diets containing varying amount of calcium from 0.01 to 10% for 14 days. The resultant changes of parafollicular cells of the thyroids of these rats were histologically investigated and their number was counted, along with the bioassay of thyrocalcitonin. The parafollicular cells were divided into the light and dark cells. Upon feeding rats with a diet containing a moderate amount of calcium, 1.0 or 2.0%, dark cells proliferated and formed

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... osing cords surrounding the follicles, and light cells decreased in number. In rats given a diet containing a high or low calcium content, light cells increased, but dark cells disappeared. A striking increase of light cells was found upon the use of a diet containing 10 % calcium. The bioassay of thyrocalcitonin only from the thyroids of these rats revealed the hypocalcemic effect. The number of parafollicular cells was not necessarily in parallel with the present bioassay data.