The effect of total parenteral nutrition (TPN) on drug conjugation in male Sprague-Dawley rats was examined using a nutrition solution composed of amino acids and glucose. The overall disposition of acetaminophen including the formation kinetics of the sulfate and glucuronide metabolites was used as an in vivo probe. Selected drug metabolizing enzyme activities also were examined in vitro. TPN, 200 kcal/kg/day, was administered by continuous i.v. infusion for 14 days and changes elicited were compared to control animals allowed free access to rat chow. TPN decreased the total clearance of acetaminophen by 34% and the formation clearance to acetaminophen sulfate by 47%. The formation clearance of acetaminophen to acetaminophen glucuronide was unaffected by TPN. Cytochrome P450 concentration and oxidative demethylase activity toward p-nitroanisole were decreased in parallel, 47 and 53%, respectively, and UDP-glucuronosyltransferase activity with p-nitrophenol and acetaminophen as the acceptor aglycones was decreased 44 and 25%, respectively in the animals receiving TPN. Sulfotransferase activity toward both p-nitrophenol and acetaminophen decreased 28% in animals receiving TPN vs. ad libitum rat chow. Administration of the parenteral nutrition solution as a continuous enteral infusion via a doudenal catheter slightly decreased p-nitroanisole demethylase activity (26%), but had no other significant effects on either cytochrome P450 concentration or on drug conjugating enzyme activities determined in vitro. These results show that parenteral nutrition administered i.v. depresses drug conjugation and suggest that alterations in both hepatic oxidative and conjugative biotransformation arising from total parenteral nutrition are largely attributable to bypassing the intestinal route for nutrient intake.