Objective: To investigate the effect of NG-nitro-L-arginine methyl ester(L-NAME), a nonselective nitric oxide synthase(NOS) inhibitor, and 7-nitroindazole(7-NI), a selective neuronal NOS inhibitor, on oxidative stress and tissue damage in brain and liver and on DNA damage of peripheral blood lymphocytes in malathion intoxicated rats. Methods: Malathion(150 mg/kg) was given intraperitoneally(i.p.) along with L-NAME or 7-NI(10 or 20 mg/kg, i.p.) and rats were euthanized 4 h later. The lipid peroxidation product malondialdehyde(MDA), nitric oxide(nitrite), reduced glutathione(GSH) concentrations and paraoxonase-1(PON-1) activity were measured in both brain and liver. Moreover, the activities of glutathione peroxidase(GPx) acetylcholin?esterase(ACh E), and butyrylcholinesterase(BChE), total antioxidant capacity(TAC), glucose concentrations were determined in brain. Liver enzyme determination, Comet assay, histopathological examination of brain and liver sections and inducible nitric oxide synthase(iNOS) immunohistochemistry were also performed. Results:(i) Rats treated with only malathion exhibited increased nitric oxide and lipid peroxidation(malondialdehyde) accompanied with a decrease in GSH content, and PON-1 activity in brain and liver. Glutathione peroxidase activity, TAC, glucose concentrations, ACh E and BCh E activities were decreased in brain. There were also raised liver aspartate aminotransferase(AST) and alanine aminotransferase(ALT) activities and increased DNA damage of peripheral blood lymphocytes(Comet assay). Malathion caused marked histopathological changes and increased the expression of iNOS in brain and liver tissues.(ii) In brain of malathion-intoxicated rats, L-NAME or 7-NI resulted in decreased nitrite and MDA contents while increasing TAC and PON1 activity. Reduced GSH and GPx activity showed an increase by L-NAME. ACh E activity increased by 20 mg/kg L-NAME and 10 mg/kg 7-NI. ACh E activity decreased by the higher dose of 7-NI while either dose of 7-NI resulted in decreased BCh E activity.(iii) In liver of malathion-intoxicated rats, decreased MDA content was observed after L-NAME or 7-NI. Nitrite level was unchanged by L-NAME but increased after 7-NI which also resulted in decreased GSH concentration and PON1 activity. Either inhibitor resulted in decreased liver ALT activity.(iv) DNA damage of peripheral blood lymphocytes was markedly inhibited by L-NAME or 7-NI treatment.(v) iNOS expression in brain and liver decreased by L-NAME or 7-NI.(vi) More marked improvement of the histopathological alterations induced by malathion in brain and liver was observed after 7-NI compared with L-NAME. Conclusions: In malathion intoxicated rats, the neuronal NOS inhibitor 7-NI and to much less extent L-NAME were able to protect the brain and liver tissue integrity along with improvement in oxidative stress parameters. The decrease in DNA damage of peripheral blood lymphocytes by NOS inhibitors also suggests the involvement of nitric oxide in this process. Objective: To investigate the effect of NG-nitro-L-arginine methyl ester (L-NAME), a nonselective nitric oxide synthase (NOS) inhibitor, and 7-nitroindazole stress and tissue damage in brain and liver and on DNA damage of peripheral blood lymphocytes in malathion intoxicated rats. Methods: Malathion (150 mg / kg) was given intraperitoneally (ip) along with L-NAME or 7-NI / kg, ip) and rats were euthanized 4 h later. The lipid peroxidation product malondialdehyde (MDA), nitric oxide (nitrite), reduced glutathione (GSH) concentrations and paraoxonase- 1 liver. Moreover, the activities of glutathione peroxidase (GPx) acetylcholinosterase (ACh E), and butyrylcholinesterase (BChE), total antioxidant capacity (TAC), glucose concentration were determined in brain. Liver enzyme determination, Comet assay, histopathological examination of brain and liver sections and inducible nitric oxide Results: (i) Rats treated with only malathion carrying increased nitric oxide and lipid peroxidation (malondialdehyde) accompanied with a decrease in GSH content, and PON-1 activity in brain and liver. Glutathione peroxidase activity , TAC, glucose concentrations, ACh E and BCh E activities were decreased in brain. There were also raised liver aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activities and increased DNA damage of peripheral blood lymphocytes (Comet assay). histopathological changes and increased the expression of iNOS in brain and liver tissues. (ii) In brain of malathion-intoxicated rats, L-NAME or 7-NI resulted in decreased nitrite and MDA contents while increasing TAC and PON1 activity. Reduced GSH and GPx activity showed an increase by L-NAME. ACh E activity increased by 20 mg / kg L-NAME and 10 mg / kg 7-NI. ACh E activity decreased by the higher dose of 7-NI while either dos(iii) In liver of malathion-intoxicated rats, decreased MDA content was observed after L-NAME or 7-NI. Nitrite level was unchanged by L-NAME but increased after 7- NI which also resulted in decreased GSH concentration and PON1 activity. Either inhibitor resulted in decreased liver ALT activity. (Iv) DNA damage of peripheral blood lymphocytes was markedly inhibited by L-NAME or 7-NI treatment. (V) iNOS expression in brain and liver decreased by L-NAME or 7-NI. (vi) More marked improvement of the histopathological alterations induced by malathion in brain and liver was observed after 7-NI compared with L-NAME. Conclusions: In malathion intoxicated rats, the neuronal The decrease in DNA damage of peripheral blood lymphocytes by NOS inhibitors also suggests the involvement of nitric oxidase e in this process.