: An electron microscopic immunocytochemical study was performed in order to determine the expression pattern of endothelial nitric oxide synthase (eNOS) in rat neurohypophysis after ischemia. In basal conditions eNOS was found to be weakly expressed on the endothelial cells and on single activated neurohypophyseal mastocytes. After cerebral ischemia, the number of mast cells increased in the neurohypophysis. The eNOS immunolabelling of mast cells was strongly enhanced between 10 min and 3 h after ischemia and declined at 24 h after ischemia. eNOS labelling was also enhanced in endotethelial cells between 30 min and 3 h after ischemia. Ultrastructurally, eNOS labelling was restricted to the granules of activated mast cells and the cytoplasm of endothelial cells. This study suggests that cerebral mastocytes are an important source of eNOS in neurohypophysis during ischemia and contribute to nitric oxide production in the perivascular space.