Inhabitants of many severe endemic goiter areas have low serum T4 and high circulating TSH, despite normal levels of T3. This situation may be produced experimentally chronically feeding rats a low iodine diet (LID). We fed rats a Remington-type LID and gave them 1% NaClO4 in their drinking water for 2 days. After this, the animals were divided into three groups. One group was fed LID, supplemented with 5 micrograms I/rat.day and was used as the control group. Another group was fed LID alone. The third group was fed LID and given 1% NaClO4 to drink. The latter treatment was used to induce severe hypothyroidism. Animals were killed 1, 2, 3, and 5 weeks after the onset of these treatment schedules. The following measurements were made on some or all groups of animals: body and thyroid weights; thyroidal I content; soluble labeled iodoprotein profile; thyroidal labeled iodoamino acid distribution pattern; plasma T4, T3, and TSH; pituitary GH content; and liver intramitochondrial alpha-glycerophosphate dehydrogenase and cytosolic malic enzyme activities. T4 and T3 concentrations were also measured in liver nuclei of the animals killed 5 weeks after the onset of treatment. As assessed from various indices of thyroid function, the LID rats became iodine deficient, although not as markedly as those given LID and ClO4-, The plasma T4 decreased to undetectable levels, and plasma TSH increased, whereas circulating T3 remained normal throughout in the LID rats. In rats given LID and ClO4-, plasma T4 decreased sooner than in rats given LID alone; plasma T3 levels also became undetectable, and TSH increased more markedly and sooner than in rats given LID alone. As measured at the end of 5 weeks of treatment, pituitary GH content, and liver alpha-glycerophosphate dehydrogenase and malic enzyme activities were lower in rats given LID than in the euthyroid LID- and I--treated controls. They were not, however, as markedly reduced as in the severely hypothyroid LID- and ClO4--treated rats. In spite of normal plasma T3 levels, the concentration of T3 in liver nuclei of the rats given LID alone was significantly lower than that of the LID- and I--treated controls. The results show that the thyrotrophs, somatotrophs, and livers of rats given LID alone are not like those of euthyroid rats despite normal circulating T3 levels. In iodine-deficient rats, there is a discrepancy between the measured indices of thyroid hormone action in the liver and the circulating T3 level, but not between biological activity and liver nuclear T3 concentration. It remains to be seen whether the same is true in the anterior pituitary.