Experimental factors implicated in the pathogenesis of halothane hepatotoxicity in the phenobarbital–hypoxia rat model were examined for direct effects on the energy status of isolated rat liver cellsin vitro. Intact hepatocytes were isolated after collagenase per-fusion of livers of adult male Fischer 344 rats previously treated with phenobarbital (0.1% in drinking water for 5–7 days) and/or deprived of food for 48 h. Cells were incubated in Krebs-Henseleit buffer + substrates for 10 min at steady states of energy metabolism, with extracellular PO2constant at 32, 16, or 4 mmHg ± 1% halothane. Fasting produced the largest energy deficits in incubated hepatocytes, regardless of phenobarbital treatment status, PO2value, or presence/absence of halothane. The combination of hypoxic PO2(4 mmHg) and 1% halothane shifted lactate metabolism toward lactate production, whereas hypoxia or halothane alone did not. Prior phenobarbital treatment plus hypoxia decreased adenosine triphosphate/adenosine diphosphate (ATP/ADP) and increased lactate production compared with drug treatment or hypoxia alone. We conclude that pathogenic factors that interact to produce halothane hepatotoxicity act directly and jointly on isolated liver cells to produce energy deficits within 10 min. Differences in the relative importance of pathogenic factorsin vitroandin vivosuggest that short-term, direct effects on hepatocellular energy status are not solely responsible for halothane hepatotoxicity.