Abstract: Research was performed to evaluate the shortterm injury of thyrod C cells and the changes of serum calcitonin level and calcitonin reserve function in SD rats after the exposure to different doses of ¹³¹I. Thirty SD rats were randomly divided into three groups: a control group, a low-dose group and a high-dose group. The serum calcitonin levels and calcitonin reserve function of all the rats were measured before and a month after receiving Na¹³¹I. The thyroids were then taken for H-E staining and calcitonin immunohistochemical staining after the rats were sacrificed. The experimental data showed that, for both the low-dose group and high-dose group, the base values, peak values and the degree of increase in serum calcitonin all decreased significantly after receiving ¹³¹I, in comparison with those values before receiving ¹³¹I ( P ＜0.05). Besides, there was significant difference in the data among the three groups ( P ＜0.05). Significant decrease in the number and distribution density of C cells in the thyroids of the rats after receiving ¹³¹I was also observed in calcitonin immunohistochemical staining. All these results demonstrated that both the level of serum calcitonin and the calcitonin reserve function decreased after receiving ¹³¹I radiation and the degree of the decrease was related to the dose of ¹³¹I.