Male and female Fischer 344 rats were exposed to 0, 1250, 2500, or 5000 ppm methyl ethyl ketone (MEK) vapors 6 hours per day, 5 days per week for 90 days. The 90-day exposures had no adverse effect on the clinical health or growth of male or female rats except for a depression of mean body weight in the 5000 ppm exposure group. The 5000 ppm animals had a slight but significant increase in liver weight, liver weight/body weight ratio, and liver weight/brain weight ratio at necropsy. Serum glutamicpyruvic transaminase (SGPT) activity in the 2500 ppm female rats was elevated while the 5000 ppm female rats exhibited significantly decreased SGPT activity. In addition, alkaline phosphatase, potassium and glucose values for the 5000 ppm female rats were increased. Special neuropathological and routine pathological studies did not reveal any lesions that could be attributed to MEK exposure.