Four groups, each consisting of 25 male and 25 female Fischer 344 rats, were exposed 6 hours per day, 5 days per week, for 13 weeks to vapor of methyl tertiary butyl ether (MTBE) at target concentrations of 8000, 400, 800, or 0 (control) ppm. The 8000 ppm concentration was chosen as the high exposure level due to the explosive nature of MTBE. The Environmental Protection Agency in the Section 4 Consent Order agreed to limit the high exposure level to 502 of the lower explosion limit for MTBE (i.e., 8000 ppm). Monitors for toxicity were as follows: daily clinical observations; function observational batted on study weeks 1, 2, 4, 8, and 13. Motor activity on study weeks 4, 8, and 13 body weights; food consumption; hematological analyses; serum chemistry including selected adrenal hormonal analyses: organ weights; and ophthalmic, gross pathologic, and microscopic evaluations. Detailed microscopic evaluations were performed on the nervous system of selected rats (6/sex/group) following perfusion fixation of tissues.