The aim of the study was to examine the effect of manganese chloride in drinking water at 1 g/L on the plasma and brain chicks AChE activity in vivo and in vitro. At 14-day-old manganese caused significant decrease in the acetycholinestrase activity in the brain of chicks about 36%. While there was no significant inhibition in plasma AChE. Dichlorvos at 7 mg/kg, orally significantly inhibited plasma and brain AChE activity. Dichlorvos at 0.5 and 1 µl significantly inhibited plasma and brain cholinesterase activity in vitro, while manganese not affected on the cholinesterase activity in vitro. manganese with dichlorvos caused decrease in ration of AchE inhibited at 7 days old in brain and plasma AChE to 59% and 69% respectively, while increased in the ratio of inhibition at 14 days old in brain and plasma AChE activity of chicks to 64% and 91% respectively. The results suggested that manganese chloride caused decrease inhibition of AChE activity in brain of chicks manganese with dichlorvos causes decrease in the inhibited ratio of plasma and brain at 7 days old while causes increased the ratio of inhibition at 14 days old.