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Background The form deprivation(FD)reduces spatial contrasts and induces myopia. Nitric oxide and cyclic guanosine monophosphate(cGMP)are involved in visual signal transmission.This study investigated changes in nitric oxide synthase(NOS)activity and cGMP concentration in ocular tissues in acute and chronic form deprivation myopia.Methods Guinea pigs had one eye covered by translucent glass for 7,14 or 21 days.Untreated litter mates were used as controls.NOS activity and cGMP concentrations in the retinal,choroidal and scleral tissues of FD eyes and controleyes were analyzed by radioimmunoassay after various durations of FD.The expression of NOS subtypes was identified by immunohistochemistry.Results Myopia was successfully induced in FD eyes after 14 days.Compared with control groups,the retinal NOS activity and cGMP concentrations in the FD eyes significantly increased after 14 and 21 days while the retinal NOS activity in the FD eyes was transiently suppressed by 7 days of FD.The NOS activity and cGMP concentrations of choroid and sclera in the FD eyes were higher than in the control groups at 21 days.The three isoenzymes of nitric oxide synthase were detected in the ocular tissues of guinea pigs.Conclusions The NOS activity and cGMP concentrations were upregulated after chronic FD and the retinal NOS activity was transiently suppressed at acute FD.The function of elevated NOS activity may be mediated by cGMP.