Abstract: Objective To study the effect of different lead exposure on nitric oxide(NO)levels,nitric oxide synthose(NOS)activity in brain and learning ability in mice and to explore the realationship between lead exposure level and brain damage as well as the potenital correlation between lead exposure and NOS.Methods Mice were exposed to low-level(0.3 g/L),moderate-level(1 g/L)and high2level(3 g/L)Pb acetate in deionized drinking water.Every group has 10 pregnant mice.The lead concentrations in blood and brain,the Morris water mazetest,and NO,NO S activity in the brain of the neonatal mice were determined at 7,14 and 21 days of lead exposure.Results The lead leve ls in blood and brain were increased with the lead exposure time and dosage(P<0.05).In water Morris mazetest,the mean incubation period in control group,low,mode rate,high lead exposure group were 33.8±7.8s,73.0±12.1s,85.9±12.7s,101.3±19.7s,respectively.The escapela tency was significant longer in leadexposed groups.The longer and higher dosage exposure,the poor was the learning performance(P<0.05).The NO content in control group,low,moderate,high lead exposure group were 10.27±4.28,6.07±1195,5.63±2.03,3.74±1.89μmol/gprot,and the NOS activity were 6.22±2.42,4.36±1.76,2.84±1.42,3.25±1.76U/mg·prot,resp ectively,which decreased in the brain of lead-exposed mice(P<0105).The escapela tency was negatively correlated withse rumlead,brain lead,NO content and the NOS activity(r=-0.540,P=0.000;r=-0.520,P=0.000;r=0.755,P=0.000;r=0.709,P=0.000).Conclusion There were significantly decreased NO content and NOS activity in the bra in after the lead exposure,which could be responsible for the impaired learning and memory behavior in lead exposed mice and the higher dosage and the longer time exposure showed more serious effects.