The state of serotoninergic system in thyrotoxic and euthyroid gland were compared. It was shown that in patients with hyperthyroidism (diffuse toxic goiter) serotonin and 5-hydroxytryptophan content, velocity and the latter uptake were augmented. The authors stress the role of the delay in serotonin oxidative deamination as one of the reasons for increased serotonin content in thyrotoxic thyroid gland. Possible ways of increasing intrathyroid serotonin accumulation and its role in the development of thyroid hyperfunction are discussed.