Nitric oxide synthase is a useful maker for nitric oxide's scope. Localized by either NADPH-diaphorase histochemistry or immunohistochemical methods, most nitric oxide synthase activity in the normal heart is present in endothelium along the extensive network of arteries, veins and capillaries within myocardium. This endothelial isoform of nitric oxide synthase also exists in the endocardium lining the cavities. Neuronal nitric oxide synthase appears much less prominent, although the exact amount of this isoform in the heart is uncertain. Scattered nerves and ganglion cells are localized by the histochemical methods. While there is no inducible nitric oxide synthase in the normal heart, macrophages associated with repair following various forms of cardiac damage contain this isoform. For all nitric oxide synthases, however, species variation and variability among models underscore the importance of correlative studies of structure and function.