The expression of endothelin-1 (ET-1), which has been proposed to have a potential autocrine/paracrine role, varies during the menstrual cycle, and therefore, ET-1 may be involved in the cyclic change of the human endometrium. However, neither the synthesis nor the degradation of ET-1 in the endometrium has been determined in detail. We investigated endothelin-converting enzyme-1 (ECE-1), which converts big-ET-1 to active ET-1, and neutral endopeptidase (NEP), which cleaves and inactivates ET-1 in human endometrium in vivo and in vitro. Western blot analysis demonstrated that the change in the expression of ECE-1 during the menstrual cycle differed from that of NEP in the endometrium. ECE-1 was expressed by endometrial epithelial cells, whereas NEP was predominantly expressed by stromal cells in vivo and in vitro. In conclusion, our results suggest that spacio-temporal expression of two endopeptidases, ECE-1 and NEP, involved in the synthesis and degradation of ET-1, might regulate ET-1 action in human endometrium.