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In order to investigate the influence of thyroid-stimulating hormone (TSH) on the ratio of newly synthesized thyroxine( T₄):tri-iodothyronine (T₃), hemithyroidectomy was performed on rats maintained on an iodinerich diet. One and two weeks after the operation the concentration of TSH increased in the plasma. As a result, the weight of the residual lobes and their thyroidal uptake of ¹³¹I/mg increased and significant histological signs of increased function in the remaining lobes were observed. The most prominent effect was a significant alteration of the ratio of newly synthesized T₄:T₃ in favour of T₃, both in the thyroid and plasma. Four weeks after the operation, when the residual lobes weighed 57% more than those in the controls, the alterations decreased or returned to normal. The total hormone concentration in the plasma (measured as protein-bound ¹²⁷I) and the oxygen consumption of the animals remained unchanged during the entire study. These findings support the hypothesis that alteration of the production and secretion ratio of T₄:T₃ induced by TSH might act as another regulatory factor, if a hormone deficiency originates in the peripheral cells. The results show also that changes of the T₄:T₃ ratio induced by TSH occur in animals on an iodine-rich diet.