AbstractStudies of the tissues of embryos and fetuses of rats treated with a riboflavin‐deficient, galactoflavin‐augmented diet during pregnancy were performed, and demonstrated that the activities of the succinic and DPNH oxidase systems (the terminal electron transport systems) were markedly reduced from control values during the critical period of development (days 12 thru 16 of gestation). The activities of the electron transport systems of placental tissue and maternal heart tissue were unaffected during the same period, suggesting that fetal tissue is particularly susceptible to the effects of riboflavin deficie