AB - 21-Aminosteroids have incited a great deal of interest owing to its ability to inhibit lipid peroxidation and prevent organ damage. The main mechanism by which 21-aminosteroids inhibit lipid peroxidation is similar to the naturally occurring chain-breaking antioxidant alpha tocopherols. Therefore, to determine whether 21-amino-steroids offer any advantage over alpha tocopherol, we compared their effects on an in vivo and in vitro models of renal injury. 21-Aminosteroid (U-74006 F) at 3 mg/kg or alpha tocopherol succinate at 10 mg/kg was administered intravenously once before bilateral renal ischemia and again before reperfusion. Acute administration 21- aminosteroid but not alpha-tocopherol, was attended by suppression of ischemia reperfusion-induced renal lipid peroxidation and injury. However, 4 weeks of dietary enrichment of rats with alpha-tocopherol (1000 IU/kg) was effective in suppressing these ischemia reperfusion-induced changes. In cell culture system, concurrent presence of 21-amino-steroid but not alpha- tocopherol abrogated H2O2-induced renal epithelial lipid peroxidation and injury. However, alpha-tocopherol was completely effective when cells were incubated with it for 14 h. Further, only the cells incubated with vitamin E for 14 h-but not for 1 or 3 h-had a significant increase in vitamin E content, which suggests that a delay in prompt cellular up take of vitamin E may explain for its lack of acute effects. Thus, unlike alpha-tocopherol, 21- aminosteroid appears readily and completely available for its chain-breaking antioxidant activity both in vitro and in vivo. 21-Aminosteroids may, therefore, offer a therapeutic advantage over alpha-tocopherols in acute injury settings.