The urinary excretion and serum levels of thyroxine (T4), triiodothyronine (T3) and 3,3',5'-triiodothyronine (reverse T3) was estimated in a longitudinal study of human newborns. The maternal and cord blood was also studied. Neonatal renal function was evaluated using endogenous creatinine clearance. In cord blood serum T3 was found to be lower than in maternal blood, but reverse T3 highly elevated. During the first 5 days of life serum T4 and T3 increased with maximum at 48 and 24 h in contrast to reverse T3 which remained high and then declined rapidly after 4 days. Creatinine clearance during the first 3 days of life increased from 5.3 to 21.9 ml/min/1.73 m2. In the same period the urinary T4 excretion increased from 79 to 281 ng/24 h, urinary T3 excretion from 16 to 44 ng/24 h and urinary reverse T3 from 4 to 15 ng/24 h. The renal excretion of thyroid hormones, corrected for body surface, was decreased compared to adult controls, corresponding to an immature renal function. The lack of ability to excrete thyroid hormones involved primary T3 and reverse T3 suggesting particular immaturity of tubular secretion of these hormones during the neonatal period.