Methyl-t-butyl ether (MTBE) is widely used as an octane enhancing agent in gasoline. A 13-week inhalation study was conducted in Fischer 344 rats to provide information on potential target organs and toxicity of MTBE, and to ascertain a no-observed-adverse-effect level (NOAEL) for MTBE. Male and female Fischer 344 rats were exposed to target doses of MTBE vapor of 0, 800, 4000 and 8000 ppm for 6 h a day, 5 days per week for 13 weeks: MTBE produced no mortalities. At 8000 ppm, males and females showed a decrease in body weights compared to controls. The only notable effect on clinical observation was ataxia at 8000 ppm, which was apparent during the first 4 weeks of treatment. Mild hematological and clinical chemistry changes were observed in the 8000 ppm group. At 8000 ppm, animals showed increased serum levels of corticosteroids, which suggest some stress-like effect. At necropsy, there were no treatment-related gross lesions. Absolute and relative organ weights (liver, adrenals and kidneys) were increased in both sexes at 4000 and 8000 ppm, but there were no microscopic lesions in these tissues with the exception of the kidney. Microscopic examination of other tissues revealed no effects with the exception that at 8000 ppm, male rats showed: mild increased size of hyaline droplets within the kidney, mild increase in hemosiderosis in the spleen and higher incidence of hyperplasia in the lymph nodes. The highest NOAEL was judged at 800 ppm.