The influence of hypophysectomy, triiodothyronine (T3), and thyroxine (T4) on the goitrogenic and tumorigenic activity of methylthiouracil (MTU) and 1‐methyl‐2‐mercapto imidazol (MMI) was studied in rats. Hypophysectomy effectively prevented the development of thyroid hyperplasia and adenomas in MTU‐treated animals. T3 reduced or abolished the weight loss of goitrogen‐treated animals, and prevented the development of thyroid hyperplasia, adenomas, and pulmonary metastatic nodules. The effect of T4 was less pronounced, especially in MTU‐treated animals. This hormone failed to neutralize the goitrogenic effect of MTU but it did reduce significantly the incidence of thyroid adenomas and prevented the appearance of pulmonary lesions. The biological effect of the thyroid hormones correlated with their effect on the serum levels of TSH, T3, and T4. The observations are compatible with the assumption that the tumorigenic effect of the two goitrogens depends upon a state of imbalance between TSH and T3.