Research appearing in the Journal of Neuropsychiatry (1992;4:189-94) linked chemical and pesticide exposure to violent behavior. Researchers at the New York University School of Medicine note that environmental exposure to cholinesterase inhibitors can produce aggressive and violent behavior. Serum or red blood cell count cholinesterase levels can be assessed in patients with idiopathic aggression. Four patients, who were exposed to cholinesterase inhibitors, were noted to become uncharacteristically aggressive. Two of these individuals committed murder. Three had come in contact with an organophosphate lawn pesticide while a fourth had used a carbamate based tick powder, after which the individual began displaying abnormally aggressive and violent behavior. None of these patients had psychiatric or neurologic diagnoses prior to exposure to cholinesterase inhibitors. All had jobs. None had ever been arrested or were prone to fighting or impulsive behavior. All the individuals felt very remorseful after their violent acts. Little or no provocation proceeded the acts. All 4 of the patients returned to normal following the termination of exposure to the cholinesterase inhibitor and none committed further violent or aggressive acts.