Introduction: Organophosphorus pesticides (organophosphate pesticides) are being most widely used as insecticides, and thus cause poisoning cases frequently. The organophosphorus pesticides at the early stage, such as parathion and TEPP, had powerful insecticidal effects and high toxicity for both humans and beasts, and caused poisoning accidents during spraying. Although many less toxic organophosphorus pesticides were then developed, the resistance to the pesticides was acquired by insects during their repeated use, resulting in less effectiveness of the pesticides. Therefore, the development of new pesticides has been being required. Now, about 40 kinds of organophosphorus pesticides are being commercially available in Japan | Organophosphorus pesticides by Masakatsu Sakata Introduction Organophosphorus pesticides organophosphate pesticides are being most widely used as insecticides and thus cause poisoning cases frequently. The organophosphorus pesticides at the early stage such as parathion and TEPP had powerful insecticidal effects and high toxicity for both humans and beasts and caused poisoning accidents during spraying. Although many less toxic organophosphorus pesticides were then developed the resistance to the pesticides was acquired by insects during their repeated use resulting in less effectiveness of the pesticides. Therefore the development of new pesticides has been being required. Now about 40 kinds of organophosphorus pesticides are being commercially available in Japan. They are being controlled by the Poisonous and Deleterious Substances Control Law of Japan. Various types of pesticides with various degrees of toxicity are available they are classfied into poisonous deleterious and common substances. Even with the common substances such as malathion and fenitrothion the ingestion of their large amounts for suicidal purpose causes fatalities. The fundamental structure of organophosphorus pesticides is O orS The pesticides are also structurally classified according to an element bound with the phosphorus into the phosphate type thiono type thiol type and dithiol type. Many of R1 and R2 are dimethoxy or diethoxy groups. To cope with resistant insects the alkyl moieties of R1 and R2 groups were replaced by unsymmetyrical propyl and ethyl groups respectively in some pesticides. As X structures alkyl alkoxy alkylthio aryl heterocyclic aryloxy and arylthio groups can be mentioned. Figure shows the metabolic pathways of a common thiono type organophosphorus pesticide having dialkoxy groups R1 and R2 . Organophosphorus pesticides undergo both enhancement of their toxicity and detoxification at the same time in mammals. The thiono type pesticide 1 shows almost no
