The ontogenesis of two anti-oxidative enzymes, thioredoxin (TRX) and glutaredoxin (GRX), was examined immunohistochemically in mouse embryos and fetuses at various developmental stages. They were found to be localized in various tissues, with some tissue specificity and temporal sequence. Both TRX and GRX began to be expressed in many tissues at embryonic (E) day E10 or E11 and tended to increase as the developmental stage advanced. In the heart and neuroepithelium, however, their immunoreactivity was already positive at E8.5. In some fetal organs like the liver, pancreas and kidney, TRX and GRX showed heterogeneous localization, suggesting that their expression may reflect the variable functional states of the cell. These results suggest that TRX and GRX may be associated with tissue differentiation in embryos and fetuses are involved in the acquisition of the capacity of resistance against oxygen radicals.