Female rats were fed a diet with a low iodine content (LID), or the same LID supplemented with KI, and mated. Fetuses were obtained at 17 and 21 days of gestation, or pups were killed at different ages after birth. The dams on LID were markedly iodine deficient and developed a large goiter. Their thyroidal iodine content was only 4% of that of LID + I dams. The iodine deficiency of the LID mothers was severe enough to result in very low plasma T₄ levels and in hepatic and cerebral T₃ deficiency, despite normal circulating levels of T₃. The fetuses from LID dams had low concentratins of iodine in their placentas and thyroid glands, and were deficient both in T₄ and Ts in all tissues studied, including the brain. After birth, however, suckling LID pups were able to increase the plasma T₄ to levels which were higher than those found in either LID fetuses or in adult LID progeny, although plasma T₄ was always lower than in age-paired LID + I animals. This increase in T₄ was probably due to an approximately 5-fold increase in iodine intake while suckling. Milk from LID mothers was found to contain 22% of the amount of iodine found in milk from LID + I dams, in contrast to their iodine intake, which was about 4% that of the LID + I rats. Cerebral T₃ levels were the same for LID and for LID -I-1 pups throughout most of the postnatal period of brain development. This finding might explain the difficulties encountered in obtaining an experimental model of neurological cretinism in rats. (Endocrinobgy121: 803–811, 1987)