Neurobehavioral effects induced by methyl-isobutyl-ketone (108101) (MIBK) and methyl-ethyl-ketone (78933) (MEK) were studied in 143 university students (75 females), 18 to 32 years old. The subjects were placed in an exposure chamber for 8 hours and exposed to 100 parts per million (ppm) MIBK, 200ppm MEK, or 50ppm MIBK plus 100ppm MEK vapor during the middle 4 hours of the period, a procedure intended to simulate an 8 hour work day. A 5 minute preexposure to 25ppm MEK and MIBK served as the negative control condition. Ingestion of 0.84ml/kg 95% ethanol served as the positive control condition. The subjects were evaluated on a psychomotor battery that included tests of choice reaction (CRT) and simple reaction time (SRT), visual vigilance, auditory tone discrimination plus tracking (dual task), memory scanning, postural sway, and mood. Blood and breath samples were collected and analyzed for MEK, MIBK, and ethanol. The subjects completed a questionnaire to rate perceived odor sensation and irritant effects. Small, statistically significant decrements in performance on visual vigilance, CRT, SRT, and the dual task were induced by MIBK, MEK, or MIBK/MEK in only four of 32 measures. Ethanol caused significant decrements in all tests except memory scanning and mood. The effect of ethanol was more pronounced in female subjects. Blood and breath MIBK and MEK concentrations tended to be higher in females. Combined exposure caused no interactive effects on the MIBK and MEK concentrations. Blood MEK and MIBK concentrations showed a positive linear dose response relationship only with the females on the auditory portion of the dual task and visual vigilance, respectively. All subjects reported significant odor sensations and irritant effect during exposure. The strongest odor sensation was reported during MIBK exposure, followed by MIBK/MEK and MEK. The authors conclude that exposure to MIBK and MEK vapors alone or in combination does not cause any significant neurobehavioral effects. The principal response appears to be sensory and irritant effects.