In order to investigate the effects of sulfamonomethoxine (SMM) on thyroid glands in short term administration as medical dose, a total of twenty albino rabbits (mean body weight 1,750g) were selected at random and allotted to control and experimental group. Rabbits in experiment were administered SMM of 50 mg/kg/day for 30 days, and histopathological changes of thyroid glands and the blood levels for T4 and T3 were observed every 5 days. The Results obtained were summarized as follows: 1. The thyroid follicles were decreased or obliterated by highly hypertrophic and hyperplastic epithelial cells and the newly formed small follicles contained scanty colloidal materials. These changes revealed increasing tendency in accordance with the experimental days. 2. The blood levels for T4 and T3 of experimental group manifested significantly decreased values than those of control through the experimental term consistently. Finally, above observations suggest that administration of SMM to animals leads thyroid glands to typical hypofunctional hyperplastic goiter.