The present work was designed to determine the dose at which no morphometrical changes occur in the epithelial linning of the follicles in the thyroid gland of rats experimentally injected with antithyroid drug Methylthiouracil (M T U). Twenty-five rats were used in the present experiment. They were divided into four groups A,B,C, and D of 5 animal each and were injected with (M T U) at a dose level of 0.5, 0.1, 0.05 and 0.02 mg. per kg. body weight respectively. In addition, a fifth group "E" of 5 animal was used as a control. It was found that at a dose level of 0.05 mg. (
M TU) per kg. body weight, no morphometrical changes occur in the epithelial linning of the thyroid follicles. Accordingly, a dose of 0.1 mg. (M T U) per kg. body weight is to be recommended for treatment of hyperthyroi dism or to prepare a hypothroid rat-model for comparative pathology.