Rats were injected intraperitoneally with 100mg/kg, 200mg/kg and 300mg/kg of methyl isobutyl ketone (MIBK) in a single dose, and the amount of MIBK in the expired air and in the urine were studied. One of the metabolites was identified as 4-methyl-2-pentanol (4M2P) in the urine by gas chromatography-mass spectrometry. The concentration of MIBK in the exhaled air attainend its maximum within 0.5 hour. Thereafter it decreased with a half life of 0.6 hour, and 41.1±8.7 (m±SD) % of the total amount injected was exhaled within 24 hours. The concentration of MIBK in the urine attained its maximum within 3 hours after injection. Then it decreased with a half life of 1.8 hours and 0.19±0.11 (m±SD) % of the total amount administered was excreted in 18 hours. The concentration of 4M2P in the urine attained its maximum in 3-6 hour and decreased gradually thereafter. Its half life was 3.2 hours and 0.31±0.18 (m±SD) % of the total amount was excreted in 12 hours.