Abstract: By using acetylcholine selective microelectrode technicque,a biphasic effect ofxylazine on acetylcholine content of the hippocampal CA₁ region in urethane anesthetized rats wasobserved as evidenced by the fact that small doses of xylazine2.0 and 6.0 mg·kg^-1)significantlyincreased,while higher dose(10.0 mg·kg^-1 )decreased the acetylcholine content.Though idazoxan(0.6 mg·kg^-1 ) significantly antagonized the effect of xylazine when used in combination,theacetylcholine content in the hippocampal CA₁ region of urethane anesthetized rats was still much lowerthan that of the control group.In rats whose nucleus locus coeruleus was chemically lesioned,xylazine(2.0 and 6.0 mg·kg^-1)significantly decreased the acetylcholine content,and idazoxan(0.6mg· kg^-1),which increased the acetylcholine content in the hippocampal CA₁ region when usedalone,completely antagonized the effect of xylazine,These results suggest that the biphasic effect ofxylazine and the lower than normal acetylcholine content oherved when idazoxan was used toantagonize xylazine induced changes in acetylcholine content of the hippocampal CA₁ region inurethane anesthetized rats might be related to the effects of xylazine and idazoxan on the centralnoradrenergic neurotransmitter system.