To evaluate the inhalation toxicity to rats of exhaust at low concentration for longer periods, Fischer 344 rats were exposed to 3 concentrations of exhaust generated by an M85 methanol-fueled engine (methanol with 15% gasoline) without catalyst for 8 h/d, 6 d/wk for 4, 8, or 12 wk. Concentration- and time-dependent increase carboxyhemoglobin in the erythrocytes and decrease in cytochrome P-450 in the lungs were observed in all treated groups. Furthermore, significant increases in plasma formaldehyde were observed in all treated groups. Furthermore, significant increases in plasma formaldehyde were observed in the group exposed to the highest concentration of exhaust (carbon monoxide, 89.8 ppm; formaldehyde, 2.3 ppm; methanol, 8.1 ppm; nitrogen oxides, 22.9 ppm; nitrogen dioxide, 1.1 ppm) for 8 or 12 wk. No change of plasma folic acid was observed in any group, and no methanol or formic acid was detected in the plasma in any animals. Histopathologically, exposure-related changes were found only in the nasal cavity of the high-concentration group. Slight hyperplasia/squamous metaplasias of the respiratory epithelium lining the nasoturbinate and maxilloturbinate were observed after 4 wk of exposure, and the incidences and degrees of these lesions increased slightly with the exposure time. No changes were found in the olfactory epithelium of the nasal cavity. As judged by optical microscopy, the exhaust concentration with no effect on the nasal cavity under the experimental conditions was concluded to be the medium concentration level containing 0.55 ppm formaldehyde. In the present study, however, concentration- and time-dependent increase of carboxyhemoglobin in the erythrocytes and decrease of the lung P-450 level were observed. Therefore, further study on more long-term inhalation of lower concentrations of exhaust might be needed.