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Effective thyroxine ratio (ETR) and total serum thyroxine (T4) estimations have been performed on 119 healthy infants divided into three groups according to birthweight and gestational age (normal, small-for-age, and preterm). ETR and T4 were determined at 1 and 5 days and at 1, 3, 6 and 12 months. During the first month the values of both ETR and T4 were significantly higher than at the age of 3 months and later. This marked postnatal spurt of thyroid activity, which is probably due to increased secretion of the thyroid-stimulating hormone, was slightly lower in small-for-age and preterm infants than in fullterm ones. From the age of 3 months onwards the values of both ETR and T4 were similar to those of adults.