Acetylcholinesterase (AChE) activity in the cerebral cortex, cerebellum, hippocampus, hypothalamus, midbrain, pons and medulla oblongata of mature male rats was determined in control and cyanide-treated animals by a spectrophotometric method. Compared with control, cyanide-treated animals showed significant increases in the AChE activity of the cerrebral cortex, hippocampus and midbrain. This finding suggests an involvement of brain AChE in acute cyanide toxicity.