Vitamin A (V.A) is one of the essential nutrients for living bodies. Based on epidemiological studies, it has been reported that those whose diets include V.A and carotenoids have a lower risk of getting cancer.β-Carotene (β-C) is a useful supplier of V.A in vivo. It also has the antioxidative ability of scavenging singlet oxygen.Further, more it has recently attracted special attention because of its immune activity.In this study, we have attempted to compare the antioxidative abilities in vivo between β-C and canthaxanthin (CANT).The former is well-known as provitamin A and is readily converted to V. A in vivo, while the latter has not yet been clarified as to whether its reductive conversion to V.A takes place or not especially in mammals.After 42 days on a VA deficient diet, rats were fed for 21 days on a diet containing β-C or CANT.In order to compare the results with or without an accelerator for carotenoid absorption, the investigation was also carried out in the presence or absence of sodium cholate (cholic acid). After the experimental period, the body weights and the amounts of carotenoids, retinol, RBP (retinol binding protein), lipid peroxide, and α-tocopherol in the serum and in the liver were determined for all the rats.The reduced body weights of the rats fed on the V.A-deficient diet increased remarkably when on a diet containing β-C and compared to those of the control group without the addition of any carotenoids. However, the increase was slight for the group fed the diet containing CANT.With regard to the acceleration effect of cholic acid, the effect was significant in the β-C group, but it was hardly observable in the CANT group.The production of the lipid peroxide in the serum and in the liver was suppressed by the diet containing β-C or CANT.Based on all the findings the following conclusions were determined:(1) CANT hardly converts to retinol in mammals, (2) Both β-C and CANT have an antioxidative ability in vivo.The antioxidative activity against lipid peroxidation inhibitors of CANT was found to be stronger than that of β-C, and (3) Further investigations using other animals are necessary to confirm these conclusions.