Beta-carotene has been shown to exhibit a good radical-trapping antioxidant activity in vitro. We were interested to see if dietary beta-carotene in combination with various intake levels for vitamin A would also inhibit lipid peroxidation.Sixty male Wistar rats received vitamin A (as retinyl palmitate) for 14 weeks in the diet (40,000, 4000 and 400 IU/kg food). In the last 5 weeks one half of each group received beta-carotene (50 mg/kg food). Lipid peroxidation (induced by 10 microM Fe2+ and 0.2 mM ascorbate) was measured ex vivo in liver microsomes.The beta-carotene-treated group had similar beta-carotene levels in liver microsomes (3.4 nmol per mg protein) as the other group, irrespective of vitamin A intake. No difference in lipid peroxidation was seen between the groups with different beta-carotene and vitamin A diets.Beta-carotene is not effective in vitro as antioxidant in liver microsomes of rats fed beta-carotene with various intakes of vitamin A.