The purpose of this study was to measure serum T-4, free T-4, TSH, T-3, rT(3), T-4 sulfate, and thyroxine binding globulin at four time points within the first 24 h of life ( cord and 1, 7, and 24 h) in infants between 24 and 34 wk gestation. The infants were subdivided into gestational age groups: 24 - 27 wk ( n = 22); 28 - 30 wk ( n = 26); and 31 - 34 wk ( n = 24). The TSH surge in the first hour of postnatal life was markedly attenuated in infants of 24 - 27 wk gestation [ 8 compared with 20 ( 28 - 30 wk) and 23 mU/ liter (31 - 34 wk)]. T-4 levels in the most immature group declined over the first 24 h, whereas levels increased in the more mature groups [ mean cord and 24-h levels: 65 and 59 (NS) vs. 70 and 84 (P < 0.002) vs. 98 and 125 (NS) nmol/liter]. Free T-4 and T-3 showed only small, transient increases in the most immature group and progressively larger and sustained increases in the other gestational groups. rT(3) and T-4 sulfate levels in cord serum were higher in the most immature infants, and in all groups levels decreased initially and then variably increased. The features of a severely attenuated or failed hypothalamic-pituitary-thyroid response to delivery critically define this 24- to 27-wk group as distinct from more mature preterm infants.