There are conflicting results in the literature concerning the effect of gavage vehicle, corn oil (CO) versus aqueous suspension, on the toxicity of haloalkanes. The purpose of our study was to assess the influence of oral dosing vehicle on the acute hepatotoxicity of CCl4and nephrotoxicity ofCHCl3. Male Sprague-Dawley rats, fed ad libitum, were treated (po) with single doses of CCl4or CHCl3using corn oil (CO), or an aqueous preparation (5%) of Emulphor (EL620) or Tween-85 (Tw-85) as vehicle (10 ml/kg). Rats were killed 48 h after treatment. Blood was collected for plasma alanine aminotransferase (ALT) determination and renal cortical slices were prepared for p-aminohippuric acid (PAH) incorporation. The comparison, between gavage vehicles, of the slopes and ED50 of the dose-response curves, although not significantly different, indicated clear trends for enhanced potency with CO for CHCl3nephrotoxicity but not for CCl4hepatotoxicity. However, ALT values, a measure of the severity of effect for CCl4also indicated that CO, when compared to EL620 and Tw-85, tended to enhance CCl4hepatotoxicity at low toxicity incidence. Furthermore, CO clearly enhanced the severity of effect for CHCl3nephrotoxicity, as measured by the slice-to-medium PAH ratios, at high dosage. The greater severity of the lesion produced by exposure to these chemicals, when administered in CO, is consistent with the trends observed for their potency (dose-response curves). Our results agree with an increased toxicity of haloalkanes by the gavage vehicle CO reported in the literature. Thus, CO should be considered a potential confounder in hepato- and nephrotoxicity assays.