Incorporation of /sup 55/Fe and of (/sup 14/C) glycine was studied in control embryos and mothers and in those which had received lead in the diet from day 7 of pregnancy. Incorporation of Fe into heme of embryonic liver which increases markedly for controls on day 17 of pregnancy was depressed greatly and showed no such increase in lead-intoxicated embryos. These embryos were retarded in growth but had normal heme concentrations in body and liver. Incorporation of glycine into embryonic heme and proteins was not affected. Data on incorporation in the mothers are also presented. It is thought that the impaired synthesis of heme in lead-intoxicated embryos limits their body growth during the late phase of pregnancy.