In 19 full term newborn boys serum levels of thyroxine (T4), triiodothyronine (T3), thyrotropin (TSH) and thyroxine binding globulin (TBG) were determined on the first and fifth day of life using radioimmunoassays. T4 and T3 were also measured in 24-h urine collections as well as total iodine in urine and in breast milk. A significant decrease occurred in serum concentrations of T4, T3 and TSH from day 1 to 5 while TBG remained unchanged. At the same time an increase of total iodine (1.7 to 4.1 micrograms), T4 (35.7 to 141 ng) and T3 (44.9 to 69.9 ng) was observed in the 24-h urine assay from day 1 to 5. The hormone concentration in urine was higher in the first urine excretion than in the total first day collection. The 24-h urine assay of the fifth day showed results which were similar to the T3 measurements in the 16th to 52nd day urine analysis in neonates of a control group. Breast milk iodine intake of newborn infants was similar to the excreted urine iodine concentration. Breast milk therefore provides an adequate intake of total iodine in euthyroid neonates.