Acetylcholinesterase (AchE, EC 22.214.171.124) activity was determined in cerebral cortex, hypothalamus, adenohypophysis and adrenal gland in response to acute and chronic stress. Chronic exposure of animals to cold stress (at 4 °C for 7 days) resulted in significant decline of AchE activity in all tissues studied. Similar results were obtained when animals were exposed to acute immmobilization and cold stress (at 4 °C) simultaneously. In another experiment, animals were treated with 2 mg/kg of corticosterone prior to AchE determination. Corticosterone administration resulted in a significant decline in AchE activity of the cortex, the hypothalamus and the adrenal but failed to affect the adenohypophysis AchE level. Exposing adrenalectomized animals to acute stress resulted in no significant changes in the cortex and the hypothalamus but caused a significant decline in AchE of the adenohypophysis. It was concluded from this study that corticosterone might mediate the stress effect on AchE activity.