We examined cellular and subcellular localization of endothelial nitric oxide synthase by using immunoelectron microscopy. The results showed that immunoreaction products were associated with membranous structure, likely the endoplasmic reticulum and cytoplasmic vesicles. These membranous localization of immunoreactivities at electron microscopic level corresponded to the perinuclear granular structures observed by light microscopic immunohistochemistry for endothelial nitric oxide synthase as well as NADPH diaphorase histochemistry.