Abstract: To study the effects of permethrin(PM)on nitric oxide(NO) in rat brain t issues,the level of NO₂^- was measured by fluor escence spectro pho to metry and cyclic guanosine monophosphate (cGMP)concentration was determined by radio immunoassay (RIA).It was found that the activity of nitricoxide synthase (NOS)and the level of cGMP in various br ain regions were affected differently while rats being treated with 200mg/kg/d PM for different days,The activity of NOS and cGMP content in cerebral cortex,hippocampus and cerebellum were not significantly changed after 1 day treatment.The concentration of cGMP was decreased in cerebral cortex and cerebellum but enhanced in hippo campus with the activity of NOS inhibited in cerebellum after 3 days treatment.The activity of NOS was reduced in cerebral cortex and cerebellum with cGMP content in cerebellum lower than the control group after 5 days treatment.And the level of NO₂^- in various regions had no observed changes after PM treatment. These results suggest that the effects of PM on NO/cGMP system may contribute to the appearance and the development of its toxicities.