The cardiac plexus of the Wistar rat was investigated in whole-mount preparations of the atria by NADH-diaphorase staining and by the acetylcholinesterase (AChE) histochemical technique. The plexus lies over the muscular layer of the atria, dorsal to the muscle itself, in the connective tissue of the subepicardium. The plexus contains on average 975 +/- 150 neurons, occurring singly or gathered in packed ganglia. The ganglia are found regularly in certain situations, namely, cranially to the pulmonary veins (44% of total); cranially and to the left of superior vena cava (10%); in the interatrial groove (21%); to the left of the left pulmonary vein (11%); caudally to the pulmonary veins (12%) and in the wall of the coronary sinus (1%). Ganglia are never found on the auricular appendages. For the histochemical demonstration of AChE, the 'direct coloring' copper ferrocyanide method was used. Extrinsic nerves enter the atria along the superior vena cava, along the pulmonary veins and along the coronary sinus, forming ganglion-containing plexuses. From specific sites of these plexuses, nerves proceed to the ganglia located to the left of the superior vena cava, to the ventral and dorsal ventricular wall and to the wall of the right atrium, where they form a delicate plexus accompanying the muscle fibers. Most of the neurons of the plexuses displayed AChE activity in the cytoplasm though they presented different reaction intensities. The distribution of cardiac nerves from groups of neurons located at discrete sites may indicate that postganglionic nerves selectively project to and thus control specific cardiac regions and/or functions.