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T he mechanisms that maintain relative uterine quiescence during pregnancy remain largely unknown. A possible role for nitric oxide has recently emerged, however, the expression of nitric oxide synthase within human myometrium at midgestation, a time when the uterus is normally quiescent, has not been investigated. The purpose of this study was to identify cell types in human myometrium that contain inducible nitric oxide synthase (iNOS), and to examine changes in its expression during pregnancy and labor. We found that iNOS is expressed in smooth muscle cells of pregnant myometrium. Expression of iNOS was highest in myometrium of preterm not-in-labor patients. At term, iNOS expression fell by 75%, and was barely detectable in preterm in-labor or term in-labor specimens. There was no staining in the myocytes of nonpregnant myometrium. Western blotting also revealed a similar pattern of changes in iNOS expression. In summary, iNOS expression in the myocytes of human myometrium is increased greatly during pregnancy, and declines towards term or with labor. Significantly, preterm inlabor patients also had a large decline in iNOS expression. These data suggest that changes in myometrial iNOS expression may participate in the regulation of uterine activity during human pregnancy.