Large doses I¹³¹ equivalent to the dose of treatment for human hyperthyroidism, thyroid cancer, and 5 times the latter were given to rats in order to evaluate their effects on hemopoietic function. The results are as follows. 1) In rats given I¹³¹ equivalent to the doses for human hyperthyroidism once (100 μc/kg), no changes were observed in peripheral blood pictures (Hb, WBC, Platelet), bone marrow function (marrow nucleated cell count, Fe⁵⁹ disappearance half time) and I¹³¹ thyroidal uptakes. 2) No alterations were noted in peripheral blood pictures and bone marrow function in rats given I¹³¹ of equivalent to the doses for human thyroid cancer once (2 mc/kg) although I¹³¹ thyroidal uptakes were decreased. 3) In rats given I¹³¹ of the doses equivalent to 5 times dosage given for human thyroid cancer once (10 mc/kg), Fe⁵⁹ disappearance half time was considerably prolonged suggesting depressed marrow function, although peripheral blood pictures and marrow nucleated cell counts revealed no significant changes. Also I¹³¹ thyroidal uptakes were decreased.