Objective To investigate the relationships between fetal neural tube defects (NTD) and maternal plasma homocysteine (Hcy) level,total nitric oxide synthase (NOS) and inducible NOS (iNOS) activity.Methods A total of 28 pregnant women with clinically or prenatal ultrasound diagnosed NTD offspring in Shihezi City and its surrounding areas of the Xinjiang Uygur Autonomous Region from January 6,2008 to May 23,2011 were enrolled as NTD group.Another 41 normal pregnant women,matched in corresponding area and period,were enrolled as the control group.The plasma level of Hcy was determined by rate method,and the activities of total NOS and iNOS were detected by spectrophotometry.Linear correlation analysis and t-test were used as statistical method.Results The plasma level of Hcy in NTD group was higher than that in the control group with statistically significant difference [(15.2±7.9) μmol/L vs (8.4±3.7) μmol/L,t=4.269,P<0.01].The activity of total NOS was lower in the NTD group than that in the control group [(20.0±9.0) U/ml vs (26.0±4.1) U/ml,t=3.258],so was the activity of iNOS [(7.5±2.9) U/ml vs (12.0±4.5) U/ml,t=4.670],the differences were statistically significant (both P<0.01).Linear correlation analysis showed a negative correlation between the level of Hcy and the activity of total NOS (r=0.325,P<0.01),but no correlation was found with iNOS (r=0.205,P>0.05).Conclusions High level of Hcy and low activity of NOS and iNOS might be correlated with fetal NTD. Key words: Neural tube defects; Nitric oxide synthase; Homocysteine