The effects of diquat-induced redox cycling on the levels of cellular ascorbic acid and alpha-tocopherol were investigated in isolated rat hepatocytes. In untreated hepatocytes, the metabolism of 1 or 2 mM diquat resulted in the depletion of cellular ascorbic acid and glutathione, but not of alpha-tocopherol, in association with the induction of cell death during the experimental period. In 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) pretreated cells, 1 mM diquat induced cell death accompanied by glutathione was rapid (to 9% of controls by 15 min) and cell ascorbate was completely consumed by 2 hr of incubation. In contrast, cellular alpha-tocopherol levels were stable for the first 30 min, but were depleted in association with the onset of lipid peroxidation. Supplementation of 0.1 or 1.0 mM ascorbic acid in the incubation medium delayed the onset of diquat-induced alpha-tocopherol loss, lipid peroxidation and cytotoxicity. When the concentration of exogenous cellular ascorbic acid was consumed to below that of endogenous ascorbic acid, alpha-tocopherol loss and lipid peroxidation were initiated. The results indicate that untreated hepatocytes have an effective multicomponent antioxidant system against diquat-induced oxidative stress. However, when glutathione is depleted from hepatocytes by treatment with BCNU and diquat, ascorbic acid plays a vital role in maintaining cellular alpha-tocopherol levels and survival of the cell.