Women pass on low levels of hazardous environmental contaminants and naturally occurring substances during pregnancy and nursing. In addition to estrogenic activity, these substances exhibit antiestrogenic, antiandrogenic, and androgenic actions, and because they can affect thyroid metabolism, they are described as endocrine-active substances (EAS). Specific topics related to EAS metabolism by mothers, fetuses, and infants are discussed. There is strong evidence that the assumed risk of EAS in early human development is exacerbated by interrupting neonatal priming provided by breastfeeding. The benefits of breastfeeding are fundamental to attenuate possible EAS effects on infants due to long-term intrauterine exposure. Breastfeeding is a guarantor of health benefits, whereas its alternative (infant formula) is a predictor of some health limitations. Mothers concerned with exposure to environmental contaminants should be advised of the proven benefits of breastfeeding and the possible health limitations of formula feeding.