Adult male Wistar rats were administered 0.1, 1, 2 or 5 mCi/kg I-131 i.p. Activities were measured in the thyroid gland or region, blood plasma (total and protein bound), salivary gland, cerebral cortex (= brain), skin, liver, spleen, small intestine, kidney, heart and skeletal muscle after 1-30 (36) days. Beginning 8 days after radiothyroidectomy the remaining or possibly regenerated thyroid function was tested with a test dose of 5 microCi/kg i.p. After administration of 0.1 mCi/kg I-131 the thyroid gland shows a regeneration within 30 days, but not after treatment with the higher doses. The time course of tissue distribution of activity is dose dependent: after 0.1 mCi/kg I-131 increasing activities after the 3rd-8th day are observed in skin, brain, salivary gland, intestine, spleen, kidney, heart and skeletal muscle. After 1 mCi/kg I-131 only in skin, brain, liver and intestine increasing activities between the 8th and 30th day have been observed. After 5 mCi/kg I-131 only in skin, spleen and intestine increasing activities after the 15th day were measured. Thyroid gland activity 24 hs after administration of 5 microCi/kg I-131 showed during the experimental period of 50 days considerable variation with a factor of 4 for thyroid gland, 8 for protein bound and about 10 for total plasma I-131 activity.