The anterior branch of the left coronary artery of the rat heart was ligated and the effects of anoxia on adrenergic nerves were studied histochemically using formaldehyde-induced fluorescence for localization of norepinephrine. Greenish catecholamine fluorescence was associated in the normal or infarcted myocardium only with adrenergic nerves. Constant but not prominent changes were seen in adrenergic nerve fibers 2 or 4 h after ligation of the coronary artery; the number of dilicate adrenergic fibers was reduced and some diffusion of the histochemical reaction was seen in small areas of the infarcted myocardium. Strong effects of anoxia were seen 8 h or more after ligation of the coronary artery. These were characterized by a prominent diffusion of the histochemical reaction and gradual disappearance of adrenergic structures in 2 to 4 days of anoxia. During the healing phase the appearance of numerous adrenergic nerve fibers was closely connected with the ingrowth of a new vascular bed into the infarction area. The observations indicate that adrenergic nervous structures of the myocardium resist the effects of anoxia, which may indicate the viability of sympathetic nerves in the infarcted area during long periods of anoxia.