Using radioactive I as an indicator, the collection from injs. of 0. 1 to 100 mg of I by rabbit thyroids was studied as a function of thyroid size, type of thyroid stimulation, and time of collection. The initial collection of I is taken up in a time detd. by the time required for the injected material to reach the thyroid, i. e., less than 15 mins. % collection is greater from a small dose than from a large dose, with the exception of thyroids of the type produced by cyanide stimulation, which appear to exhibit a threshold effect. Thyroids stimulated by thyrotropic hormone or by exclusive cabbage diet take up more than normal thyroids at all values of inj. Thyroids stimulated by certain other thyrotrophic hormone preps. or by cyanide take up more than normal thyroids from large injs., less from small. Collection from one dose of I inhibits collection from a 2d in all types of gland. Successive doses were differentiated by the use of different radioactive isotopes as labels. The large relative conc. of I by the thyroid suggests the feasibility of using radioactive I as a means of administering internal radiation.