At a stage when histological damage detectable witha light microscope is slight, various indices of the thyroid function in rats immunized with total homologous thyroid extract are impaired, suggesting the existence of a ‘pre histological’ stage of isoimmune thyroiditis. The data indicate alterations of iodine utilization and hormone secretion, since the radioiodine fixation curve, iodinated amino-acids and iodothyronines concentrations in the thyroid, as well as proteolytic activity and protein bound iodine in serum, are significantly affected.