Heart rate, performance, and symptoms were studied in six female and six male volunteers, aged 19 to 47 years, during experimental 2-hr exposures to 10 and to 200 mg/m3 of methyl isobutyl ketone (MIBK). No effects from exposure on performance of a reaction time task or an arithmetic test could be demonstrated, and no consistent effects on heart rate were found. Subjects reported significantly more symptoms from the central nervous system, e.g., fatigue, due to the exposure. There was also an indication of an increase in ratings of irritation to the airways. A reduction of the threshold limit value (TLV) of 205 mg/m3 for MIBK exposure presently indicated by the American Conference of Governmental Industrial Hygienists, is therefore recommended.