A single 6-hr exposure to inhaled methanol (MeOH) has been shown to enhance carbon tetrachloride (CCl4) hepatotoxicity. The objective of the present study was to use gas uptake data and the development of a physiologically based pharmacokinetic model (PBPK) to determinein vivochanges in CCl4metabolism resulting from MeOH pretreatment. Adult male F344 rats (167–197 g) were exposed to 10,000 ppm MeOH (constant concentration) via inhalation for 6 hr. Individual rats were exposed using gas uptake techniques to CCl4alone or to CCl4either 24 or 48 hr after initiation of MeOH pretreatment. The following initial concentrations were used for CCl4: 0, 25, 100, 250, and 1000 ppm with exposures lasting 6 hr.Vmax(metabolic rate) was estimated from gas uptake data andKm(Michaelis constant) was assumed constant after methanol pretreatment. For CCl4alone,Vmaxwas 0.11 mg/hr (Vmaxc= 0.37 mg/hr/kg) andKmwas 1.3 mg/liter.Vmaxwas 0.48 mg/hr (Vmaxc= 1.6 mg/hr/kg) for the 24-hr MeOH + CCl4group andVmaxwas 0.18 mg/hr (Vmaxc= 0.6 mg/hr/kg) for the 48-hr MeOH + CCl4group. For CCl4alone, serum markers of hepatotoxicity alanine aminotransferase (ALT) and sorbitol dehydrogenase (SDH) were increased significantly only at 1000 ppm CCl4. Both serum markers of hepatotoxicity in the 24-hr MeOH + CCl4group increased as a function of CCl4concentration when compared with 0 ppm CCl4controls. The maximum increase occurred at 1000 ppm CCl4, where ALT and SDH increased by 392- and 286-fold, respectively. At 100, 250, and 1000 ppm CCl4, ALT and SDH values for the 24-hr MeOH + CCl4groups were significantly increased relative to control (0 ppm CCl4), CCl4alone, and 48-hr MeOH + CCl4. ALT and SDH levels in the 48-hr MeOH + CCl4groups were not statistically different from the respective CCl4alone groups.