In an attempt to better understand the mechanisms for pseudoalcoholic liver lesions in human beings, we determined the effects of perhexiline on mitochondrial functions in mice and rats. A first series of studies suggested that protonated perhexiline entered mouse mitochondria along the mitochondrial membrane potential. Release of a proton in the mitochondrial matrix led to uncoupling of oxidative phosphorylation, and accumulation of perhexiline inhibited complexes I and II of the respiratory chain, decreased ATP formation in vitro and decreased the mitochondrial beta-oxidation of long-, medium- and short-chain fatty acids in vitro and in vivo in mice. In cultured rat hepatocytes, exposure for 24 hr to 25 mumol/L perhexiline markedly decreased hepatocellular ATP and cell viability. Exposure to 5 mumol/L perhexiline did not modify ATP and viability but decreased the beta-oxidation of palmitic acid uniformly labeled with carbon 14 by 38%, increased hepatocyte triglyceride levels by 98% and produced microvesicular steatosis after 72 hr of culture. We conclude that perhexiline is concentrated inside mitochondria, where it inhibits both oxidative phosphorylation and the mitochondrial beta-oxidation of fatty acids. These effects may contribute to the development of necrosis, steatosis and possibly certain other pseudoalcoholic liver lesions in human beings.