The purpose of the investigation was to study the goitrogenic activity of milk in the endemic goitre district in Finland by feeding rats with milk produced in this district. During one to two years the size of the animals'' thyroid glands increased to almost twice the size of the thyroids of control rats given milk from the non-goitrous district. A marked excess of iodine in the food was not capable of inhibiting this goitrogenic effect. The occurrence of goitrogens in cattle fodder and milk in the goitre district may be explained, e. g., on the basis of the condition of pastures in Finland and may be responsible for the human goitre endemia in this country, especially when definite evidence of iodine deficiency in food is still lacking.