Histological, histochemical fluorescence, and electron microscopic techniques have been utilized to demonstrate the specific distribution of the noradrenergic sympathetic postganglionic fibers to the guinea pig left atrium and ventricle. The majority of these nerves entered the base of the heart. Paravascular bundles of unmyelinated sympathetic postganglionic fibers traversed the left atrioventricular sulcus. From these paravascular plexuses axons accompanied the arteries via periarterial plexuses to the deeper layers of the heart wall. The vascular tree was extremely complex. In the left atrium a relatively abundant noradrenergic innervation was seen within the interstitial connective tissue surrounding the cardiac muscle. In the left ventricle, most of the noradrenergic innervation was located in the tunica adventitia of all the major coronary arterial branches before they entered the cardiac muscle. These larger branches of the coronary arteries were predominantly innervated by the sympathetic postganglionic fibers. Apparently it is from these perivascular nerve plexuses that the left ventricular muscle receives the majority of its noradrenergic innervation.