The effect of acetaminophen on lipid peroxidation in vivo and in vitro was studied in rat liver and the data were compared with those with carbon tetrachloride. Carbon tetrachloride increased diene conjugates in vivo and thiobarbituric acid reactive substance production in vitro in the liver microsomal incubation. These changes were further enhanced by ethanol that has previously been shown to increase carbon tetrachloride-induced hepatotoxicity. On the other hand, acetaminophen did not increase diene conjugates in vivo and inhibited thiobarbituric acid reactive substance production in vitro. These effects were minimally affected by ethanol which has previously been shown to inhibit acetaminophen-induced hepatotoxicity. Thus, lipid peroxidation may play a minimal role in acetaminophen-induced hepatotoxicity in contrast with carbon tetrachloride-induced hepatotoxicity.