The known potent inhibition of microsomally catalyzed covalent binding of acetaminophen by ascorbate has led to the suggestion that ascorbate rapidly reduces the reactive metabolite of acetaminophen and hence should protect against acetaminophen hepatotoxicity. Experimentally, ascorbate given immediately after acetaminophen did not protect against hepatotoxicity in the hamster and did not suppress in vivo covalent binding of acetaminophen. Ascorbate administration resulted in a rapid and marked increase in liver levels of reduced ascorbate. Hepatic ascorbate levels in both control and ascorbate-treated animals were unaffected by the administration of hepatic doses of acetaminophen. Also unchanged by ascorbate in vivo were the overall rate of acetaminophen elimination, acetaminophen-dependent depletion of hepatic glutathione, and the amount of acetaminophen-mercapturate excreted in the urine. Since these indices reflect the hepatic formation and fate of the reactive metabolite of acetaminophen, the data indicate that increased levels of ascorbate did not enhance reduction of the reactive metabolite of the drug back to acetaminophen in vivo, as was suggested from liver microsomal studies. Further studies examined the ascorbate sensitivity of the acetaminophen covalent binding reaction in ascorbate-deficient freshly isolated hepatocytes. In contrast with both the in vitro microsomal and the in vivo binding, covalent binding in the isolated hepatocyte was partially inhibitable by ascorbate. The ascorbate-insensitive binding, which represented approximately 50% of total binding, was inhibitable by cysteine. These data indicate that microsomally catalyzed covalent binding differs significantly from the covalent binding which occurs in the intact liver.