We studied the effect of carbon monoxide(CO)-induced bypoxia on monoamine neurotransmitter and their syntheitc enzyme in rat brain. When the rats were acute or chronic intoxicated at CO 5000 ppm for 30 minutes or 2000 ppm for 1 week each 3 hours, dopamine content increased significantly with decreasing of its turnover in striatum and norepinephrine content was diminished in hypothalamus. 5-hydroxytryptamine content was increased significantly and its turnover was decreased both in striatum and hypothalamus. Tyrosine hydroxylase activity was reduced in striatum. These results suggest that inhibition of TH activity in CO-induced hypoxia is owing to lack of oxygen supply threfore NE content is decreased. We suggest that increasing of dopamine and 5-hydroxytryptamine are due to reduction of its turnover.