In studies of methyl isobutyl ketone (MiBK)-potentiated cholestasis induced by taurolithocholic acid (TLC) or manganese-bilirubin (Mn-BR) combinations, MiBK is usually given by gavage whereas industrial exposure to MiBK normally occurs by inhalation. The present study was conducted to verify if the route of administration could influence the potentiation. Male Sprague-Dawley rats were treated with MiBK for 3 days orally or by inhalation (4 hr/day). The minimal effective doses (MED) for potentiating both models of intrahepatic induced cholestasis were estimated to be 3 mmol/kg or 400 ppm for the oral or inhalation route, respectively. Groups of rats were treated with 0.5, 1, or 2 times the MED. Half of each group was sacrificed after the last MiBK administration to determine plasma concentrations of MiBK and its metabolites by gas-liquid chromatography. The other half was challenged 18 hr later with TLC (30 mumol/kg) or a combination of manganese (4.5 mg/kg) and bilirubin (15 mg/kg). Bile flow was measured from 15 to 135 min after the cholestatic challenge. Rats exposed to MiBK orally or by inhalation exhibited an enhanced diminution in bile flow that was dose-dependent. With dosages of 3 mmol/kg po or 400 ppm by inhalation or more, diminution in bile flow was significantly different from control values. Comparisons between maximal bile flow decrease and MiBK plasma concentration showed that the severity of the hepatotoxic response was dependent on the plasma MiBK concentration, irrespective of the route of administration.