OBJECTIVE: This study evaluated the role of nitric oxide in the maintenance of uterine quiescence in nonpregnant and pregnant ewes. STUDY DESIGN: Sixteen ovariectomized nonpregnant and 10 pregnant (115 days' gestation) chronically instrumented ewes were studied. Uterine contractility was assessed by electromyography and intrauterine pressure recordings. Nitric oxide synthase inhibition was induced with nitro-L -arginine methyl ester or aminoguanidine (4. 5 mg/kg per hour) given during estrogen replacement with 17beta-estradiol (100 microg/d) or in late gestation. In the pregnant group we evaluated the ability of nitric oxide synthase inhibition to alter the responsiveness to oxytocin-induced uterine contractility. Blood pressure and common internal iliac artery blood flow were assessed to confirm nitric oxide synthase inhibition. In addition, the effects of the nitric oxide donor nitroglycerin and the cyclooxygenase inhibitor indomethacin were studied in nonpregnant sheep. The effect of nitric oxide in vitro on myometrial spontaneous and induced contractions was also studied. RESULTS: In nonpregnant estrogen-replaced sheep, nitric oxide synthase inhibition and nitroglycerin administration did not alter uterine contractility, despite significant changes in blood pressure. In contrast, indomethacin decreased electromyographic results to 70% of baseline after 1 hour and 47% after 2 hours. In pregnant ewes nitric oxide synthase inhibition failed to alter uterine contractility in response to oxytocin. These findings are in contrast to results of the in vitro study in which nitric oxide was shown to relax sheep myometrium. CONCLUSION: The absence of significant effects of nitric oxide synthase inhibition and nitric oxide donors on uterine contractility in vivo suggests that nitric oxide does not play a physiologic role in the regulation of uterine contractility in nonpregnant or pregnant ewes.