1) Pregnant rats were fed 0.15% propylthiouracil (PTU) from the 10th to the 19th day of pregnancy and 1, 2, 3, 4 or 5 μg. T4 were injected daily during this time. The regression slopes of the thyroid weights were similar to those of the corresponding controls except that 2 μg. T4 produced less decrease in thyroid size in pregnant than in non-pregnant rats. 2) Pregnant rats were fed a low-iodine diet from the 10th to the 19th day of pregnancy and doses of T4 were administered during this interval as in 1). The regression slopes of the thyroidal I131 uptake of the pregnant and non-pregnant rats were identical but uptakes were consistently slightly lower at each point for the pregnant rats. 3) Following seven days of low-iodine diet, thyroidal I131 uptakes were determined at one to two day intervals from the 8th day of gestation to the 29th day of lactation. Uptakes were not statistically different from the controls until the 20th day of pregnancy when a marked drop of approximately 50% occurred which persisted throughout lactation. This decrease was correlated with a concomitant sudden rise in iodine uptake by the mammary glands. The high uptake by the mammary glands persisted throughout lactation and always exceeded that of the thyroid. Fetal thyroid uptake was negligible until the 20th day of pregnancy and then was only 1/4 that of the mother. There was no consistent change in thyroid weight. These results indicate that there is not a significantly increased demand for thyroid hormone during pregnancy in the rat. However, the marked decrease in radioiodine uptake by the thyroid during lactation seems related to the successful competition of the mammary glands for iodine during this time.