Free acetylcholinesterase activity in whole homogenate of the brain stem was assessed every 4 hours over a 48 hour period occurring in the last lunar quarter, studied every month of the year, using the Ellman's spectrophotometric method. At the same time locomotor activity was registered in experimental animals. The annual course of acetylcholinesterase activity and locomotor activity revealed a cyclic occurrence of the maximal values (every 6 months). Therefore it can be concluded that locomotor activity and acetylcholinesterase activity in the mouse brain stem are changed by the time of the year.