Endogenous nitric oxide (NO) can be synthesized by endothelial cells and can act as a potent vasodilator. We investigated endothelial nitric oxide synthase (eNOS), one of the three different enzymes responsible for the synthesis of NO by immunohistochemical methods throughout the menstrual cycle on 34 endometrial samples and compared its detection with the von Willebrand Factor (vWF) as a reliable marker molecule of the endothelium on serial sections. Immunoreactivity for eNOS was clearly localized in various types of arterial and venous endothelial cells as well as in capillaries. In addition, in some samples there was a positive staining in endometrial glandular epithelium. There was no staining in endometrial fibroblasts or in myometrial smooth muscle cells. Whereas the endothelium was constantly stained by the monoclonal antibody against vWF, eNOS was not always expressed in the endothelial lining of the vessels during the menstrual cycle. The number of vessels positively stained for eNOS increased gradually during the proliferation phase and most of the vessels were positive in the early secretory phase. These results suggest that its markedly increased expression during the early secretory phase of the menstrual cycle indicates a physiological significance.