Serum concentrations of thyrotropin (TSH), thyroxine (T4), 3,3',5-triiodothyronine (T3), and 3,3',5'-triiodothyronine (rT3) were determined in blood samples from 140 full-term healthy newborns, 110 appropriate weight and 30 large-for-gestational age, aged 1 to 6 days, delivered vaginally and breast-fed. Serum TSH levels decreased progressively from the 1st to the 4th day; serum T4 levels increased, with a peak on the 2nd day, and then progressively decreased until the 6th day; serum T3 levels increased to a maximum value on the 2nd day and then decreased to a minimum on the 5th day; serum rT3 levels increased during the 1st day and the level remained constant from the 2nd to the 4th day and later decreased slightly. The decrease of T3 was more pronounced than that of T4, while rT3 remained at high levels until the 4th day. Dividing the data into narrower intervals of time, it was possible to show that the maximum value of TSH was followed first by a net increase in serum T3, then in T4, and lastly in rT3 ant T3 levels. These data indicate that the rapid increase after birth of serum T3 levels is prevalently TSH-dependent; the following increase in serum levels of T3 and the increase in rT3 are prevalently T4-dependent. This study provides data concerning physiological changes in TSH and thyroid hormones in serum from a large number of infants, during the first week of life. They should be useful for the understanding of thyroid function in early postnatal life.