Abstract: Objective: To investigate the changes of endothelin-1 receptor(ETa-R) expression in human myometrium as well as fetal membranes during the course of labor and its relationship to the labor onset.Methods: Immunohistochemcal SABC method and computer quantitative image analysis were employed to determine the levels of ETa-R in human myometrium and fetal membrane before and during the labor.Results: Although the ETa-R was expressed in human myometrium either before or during the parturition,the positive rate and level of ETa-R were significantly higher after the onset of labor(P<0.01).In fetal membranes,the ETa-R was mainly distributed in syncytial trophoblasts and intermediate trophoblasts.The positive rate of ETa-R was increased significantly after the onset of labor(P<0.01),whereas there was no significant difference in the level of ETa-R(P>0.05).Conclusions: The Up-regulation of ETa-R expression in myometrium and fetal membrane during the course of labor suggests that ET-1 may be important in the initiation of human parturition.