In this study, the changes in acetylcholinesterase (AChE) activity in the liver, lungs, kidney and brain tissues were investigated in the rats administered aspartame intraperitoneally. Aspartame, a widely used artificial sweetener, was given to the rats at doses of 50, 100 and 200 mg/kg. Changes in enzyme activity were recorded at 12 and 24 h following the injection. The activities of AChE at 12 h for all doses were significantly higher in the liver and kidney but not in those of lung and brain. However, the activities in the liver and kidney at 24 h after treatment for 200 mg/kg were similar to that of control. Consequently, it was observed that aspartame caused increases in the activities of AChE in the liver and kidney during the first hours after treatment but lost its effect at later times.