The autonomic innervation in the heart is predominantly by postganglionic, parasympathetic, and sympathetic neurons, which are organized in a plexus of ganglions into the heart. The knowledge of the density, distribution, location, morphology, and intrinsic connection of this system that constitute the heart autonomic innervation is limited and controversial. Methods that provide clear information in this field are desirable. A widely used method to study the morphology of the nerve cells in the central nervous system (CNS) is used in this study to characterize the autonomic innervation in rat hearts. The method consisted of impregnation of the fresh whole heart of 12 adult male Wistar rats with the Golgi-Cox stain for 30 days, after which they were incubated in 30% sucrose solution for 2-3 days and then were sectioned (200 microm) with a vibratome. The tissues were mounted on gelatin-covered slides and analyzed by using the Sholl method under light microscopy adapted to a camera lucida. The results clearly show a distribution of the ganglion plexuses in the epicardium, myocardium, and endocardium, joined by an extensive network of nerve fibers in the four cardiac chambers. We also identified and characterized the morphology of an isolated cardiac nerve cell. As results similar to that in the CNS, the Golgi-Cox method is a simple, efficient, and an accessible tool to study the autonomic innervation in the rat heart and provides a good resolution of the morphology of the plexuses of the ganglions and nerve fibers.