Objective: Cervical tissue expresses all the isoenzymes of nitric oxide synthase. We studied the concentrations of nitric oxide metabolites in the cervical fluid in nonpregnant (n = 11) and pregnant women (n = 106). Study Design: Cervical fluid was collected into a Dacron polyester swab, and nitric oxide metabolites were eluted into physiologic saline solution, which was assayed for nitric oxide metabolites with the Griess reaction. The detection limit of the method is 0.2 μmol/L. Results: Cervical fluid nitric oxide metabolite was detectable in 46% of nonpregnant women (median, <0.2 μmol/L; 95% CI, 0-49), in 63% of women in early pregnancy (median, 11 μmol/L; 95% CI, 0-23) and in 82% of women in late pregnancy (median, 128 μmol/L; 95% CI, 21-276). In late pregnancy, the cervical fluid nitric oxide metabolite level was higher in women with Bishop score of ≥6 (median, 163 μmol/L; 95% CI, 105-276) than in women with Bishop score of <6 (median, 86 μmol/L; 95% CI, 21-99). Cervical fluid nitric oxide metabolite concentration before the onset of labor in parous women (median, 97 μmol/L; 95% CI, 78-283) was higher (P = .008) than that in nulliparous women (median, 28 μmol/L; 95% CI, 0-95). Cervical fluid nitric oxide metabolites before the initiation of labor (median, 33 μmol/L; 95% CI, 0-95) rose to 3.5-fold (median, 115 μmol/L; 95% CI, 78-284) after the commencement of uterine contractions and showed a significant relationship to Bishop score (r = 0.39, P = .01). Cervical fluid nitric oxide metabolite concentrations were not relative to simultaneous plasma nitric oxide metabolite levels (n = 41 women, r = 0.14, P = .41). Rupture of fetal membranes tended to decrease cervical fluid nitric oxide metabolite levels, whereas gentle cervical manipulation elevated it 6.6-fold in 1 minute. The administration of glyceryl trinitrate (0.5 mg, nitric oxide donor) intracervically resulted in a significant rise in the cervical fluid nitric oxide metabolite level in 2 minutes. Conclusion: Cervical fluid nitric oxide metabolite level rises after cervical ripening, nitric oxide donor administration, or cervical manipulation, which supports a role for cervical nitric oxide in cervical ripening. (Am J Obstet Gynecol 2003;188:779-85.)