The distribution of nitric oxide synthase(NOS)in brain tissues of rats exposed to deltamethrininsecticide has been examined by histochemical NADPH-diaphorase staining techniques on frozen sec-tions.After injection of deltamethrin(12.5mg/kg,i.p.),a reproducible sequence of toxic signs ofhyperexcitability were elicited.The observation and image analysis showed that,within brain sec-tions of rats exposed to deltamethrin,the numbers and the total staining areas of the NOS positiveneurons were greatly increased,especially in cerebral cortex,hippocampal formation and paraventric-ular nucleus.In addition,the density of single neuron and the processes were also increased.The re-sults suggested that deltamethrin may induce the NOS expression or activate the NOS activity.TheNOS activation may involve in the chains responsible for the excitatory neurotoxicities induced bydeltamethrin. The distribution of nitric oxide synthase (NOS) in brain tissues of rats exposed to deltamethrininsecticide has been examined by histochemical NADPH-diaphorase staining techniques on frozen sec-tions. After injection of deltamethrin (12.5 mg / kg, ip), a reproducible sequence of toxic signs of hyperexcitability were elicited. The observation and image analysis showed that, within brain sec-tions of rats exposed to deltamethrin, the numbers and the total staining areas of the NOS positive neurons were greatly increased, especially in cerebral cortex, hippocampal formation and paraventric- The density of single neuron and the processes were also increased. The re-sults suggested that deltamethrin may induce the NOS activity or the activation of the NOS activity.TheNOS activation may involve in the chains responsible for the excitatory neurotoxicities induced bydeltamethrin .