The response of blood plasma acetylcholinestrase to acute administration of chlorpromazine and diazepam was examined in chick. Chlorpromazine in a dose–dependent manner at 25, 50, and 75 mg/Kg, subcutaneously inhibited the activity of Ache in the blood. Chlorpromazine (50, 75 mg /kg) significantly decreased the enzyme activity in the plasma by 14.7 % and 17.6 %, respectively for a period of 60 min, as well as, 14.5 % and 13.1 % for a period of 60 and 120 min, respectively in groups received chlorpromazine 50 mg/kg. Diazepam at 10, 20 and 30 mg/kg subcutaneously did not significantly affect the enzyme activity in comparison with the control group. The result suggests that chlorpromazine changes Ache activity in the blood and possibly in other tissues.