T4, T3, rT3, and TSH were measured in cord blood of infants born in mild to severe endemic goiter areas and nonendemic urban areas in South and North America. A significantly high rT3 concentration was found in newborns living in the urban area of South America as compared to a Canadian newborn population. There were no differences among the various thyroid parameters in the groups of newborns studied. Blood from goitrous mothers showed significantly higher concentrations of T3 and TSH than nongoitrous mothers. However, these changes did not affect the concentrations of thyroid hormones, rT3, or TSH in the fetal circulation.