Grapefruit juice is a rich source of nutrients and citrus flavonoids which are proposed to provide important health benefits. Many flavonoids are good antioxidants in vitro and this property is one that is hypothesized to mediate their biological effects in vivo. In two studies we tested the effects of citrus flavanones and grapefruit juice on lipid oxidation and the antioxidant defense system in vivo. The first tested the dose-response effect of naringenin consumption in rats adequate or deficient in vitamin E and selenium on growth and multiple measures of antioxidant defense, lipid oxidation and oxidative stress. The second was a human intervention trial of three months in duration testing the effects of regular consumption of grapefruit juice on antioxidant nutrient status and oxidative stress in young adult women. No evidence of antioxidant or pro-oxidant activity was observed, however, in rats inhibition of adipose accumulation was observed.