Fetuses from rats given either water or 0.03% D,L-..cap alpha..-tocopherol acetate (vitamin E) as a drinking fluid and X-irradiated with 100 rad on gestational day 13 were examined on gestational day 21. Mean cerebral weight which was significantly reduced by the X-irradiation was increased by vitamin E supplementation but the level did not reach that in sham-irradiated controls. Administration of vitamin E caused an increase in DNA concentration which was significantly reduced by X-irradiation with water treatment. An increase in the mean level of lipid peroxide formation was observed in the water-treated, X-irradiated group in the sample at zero time but not in the vitamin E-treated, X-irradiated group. In the cytoplasm of fetal cerebral neurons from X-irradiated dams with vitamin E supplementation, confronting cisternae were frequently observed between two nuclear envelopes. Confronting cisternae may be considered as a repair mechanism of vitamin E against X-irradiated neuronal damage in the fetal cerebrum. This study provides evidence of the protection by vitamin E of neuronal development in X-irradiated fetuses, through its antioxidant properties, against attacks by free radicals and/or lipid peroxide. 35 refs.; 3 figs.; 1 table.