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p-Nitrophenol is widely employed as a substrate for the study of the glucuronide and sulfate conjugation pathways. However, in perfused livers from ethanol-treated rats, p-nftrophenol was rapidly metabo-lized to 4-nftrocatechol. The 4-nftrocatechol formed competed with p-nitrophenol for conjugation with glucuronic acid and sulfate, and interfered with the direct spectral measurement of p-nitrophenol. In isolated microsomes, rates of p-nftrophenol hydroxylation were in-creased 6-fold after chronic ethanol treatment. Much smaller induc-tion was observed after pretreatment of rats with phenobarbital(70% increase) or -naphthoflavone (30 % increase). In addition, the 6-fold increase in rates of p-nitrophenol hydroxylation after chronic ethanol treatment was greater than increases in activity observed for the microsomal metabolism of aniline, 7-ethoxycoumarin, benzo-(a)pyrene, ethanol, or aminopynne. These data demonstrate that p. nitrophenol may be an extremely useful substrate for the study o