Chronic inhalation exposure to unleaded gasoline (UG) induced an increase in liver tumors in female but not male mice. Unleaded gasoline exhibits little, if any, genotoxic activity in vitro or in vivo in the female mouse liver, suggesting that other biological effects such as the induction of cell turnover or altered growth control may play a role in this carcinogenic process. To better understand the role of UG-induced hepatocyte proliferation with respect to the dose- and sex-specific tumor response, male and female B6C3F1 mice were housed in 1-m3 single-pass flow-through inhalation chambers and administered UG under exposure conditions that produced tumors in the chronic studies. Mice were exposed to targeted concentrations of 67, 292, or 2056 ppm PS-6 blend of UG vapor 6 h/d, 5 d/wk, for up to 13 wk. Liver weights were elevated significantly in male and female mice exposed to 2056 ppm UG at wk, 1, 3, 6, and 13. No elevation in liver-specific serum enzymes was noted in treated animals, nor were there any significant histopathological changes in the liver, indicating a lack of overt hepatotoxicity. Hepatocyte proliferation, expressed as nuclear labeling index (Ll), was measured immunohistochemically after 5-bromo-2'-deoxyuridine administration via an osmotic minipump implanted three days before the animals were killed. A 6-to 10-fold increase in Ll compared to controls was observed in male and female mice exposed to 2056 ppm UG at wk 1, with a return to control levels at wk 3, 6, and 13. Mice exposed to 67 or 292 ppm UG did not show any increase in Ll. The mode by which an agent induces cell proliferation is an important consideration in mechanistic studies and the risk assessment process. These data indicate an early transient mitogenic stimulation of cell proliferation, rather than regeneration secondary to cytotoxicity, in the livers of UG-treated mice. The observed proliferative response after UG exposure in the male mouse in the absence of a tumorigenic response suggests that effects in addition to the early transient hepatocyte proliferation response are critical in understanding the sex-specific hepatocarcinogenic response of this complex mixture.