To study the pattern of thyroglobulin (Tg) serum concentrations in the first days of life, Tg, TSH, T3, and T4, were measured in both maternal and cord blood at delivery and 1, 6, 12, 24, 48, 72, and 96 h after birth in 16 newborns. Tg levels at birth wee higher in cord blood than in maternal blood. After birth, a prolonged Tg rise occurred. The increased levels became significant at 6 h and were maintained throughout the study. Tg levels were not correlated with serum levels of T3, T4, and TSH. We conclude that either a reduced Tg MCR or an increased thyroidal secretion of Tg after endogenous TSH stimulation in the newborn is a possible explanation for the elevated Tg levels occurring during the first days of life.