Prenatal or early postnatal exposure to several agents displaying hormonal action causes persistent quantitative and qualitative changes in hormone receptors in various cell-types. Exposure must occur during the windows of susceptibility, which occurs at specific times for each cell-type and hormone receptor. These alterations, that persist through life, are induced by the mechanism of epigenetic imprinting (cell programming). Studies performed in our Labs and elsewhere found that not only hormones or agents displaying hormone action, but also those not displaying this activity, may induce the mechanism of imprinting; among them, lead and arsenic.