It has been implied that the antiperoxidative activity of beta-carotene is important for its ability to prevent malignant and cardiovascular diseases. In vitro studies have shown that 9-cis beta-carotene is a better antioxidant than the all-trans isomer. In the present study the antiperoxidative biopotency of 9-cis beta-carotene was studied in vivo. For three weeks, weanling female rats were fed diets containing 10% fresh or oxidized soybean oil. The experimental diets were supplemented with beta-carotene at 1 g/kg with the synthetic all-trans isomer or with a carotene extract of Dunaliella bardawil containing 75% 9-cis beta-carotene. Both carotene sources prevented to the same extent hepatic and erythrocyte peroxidation associated with the consumption of oxidized oil. However, this beneficial effect was accompanied, in most of the groups, by a reduction in the hepatic carotene stores. Only in the animals fed Dunaliella extract combined with oxidized oil were the hepatic stores of beta-carotene and vitamin A maintained. The enhanced degradation of 9-cis beta-carotene observed in the livers of these animals might indicate that, like the effect observed under in vitro conditions, this isomer has a greater affinity toward free radicals and therefore might be a more efficient antioxidant than the all-trans form under in vivo conditions. The activity of glutathione peroxidase, glucose-6-phosphate dehydrogenase, and glutathione reductase as affected by the two carotene sources was also studied.