Physiological studies indicate that cardiac parasympathetic nerves may act selectively at discrete cardiac sites. To determine anatomical sites at which selective integration of cardiac nerve activity may occur, the present study identified and described the location, distribution, and projections of intracardiac ganglion cells in the rat. The estimated 3992 ganglion cells per rat heart were located in 4 distinct groups, all above the atrioventricular groove: (1) between the superior vena cava and aorta (2.5% of total), (2) in the region of the superior interatrial septum (49.9%), (3) posterior to the left atrium (24.0%), and (4) posterior to the inferior interatrial septum and right atrium (23.5%). Only a few ganglion cells were located subepicardially within the infolding of the dorsal interatrial septum. Retrogradely transported fluorescent tracers injected into the left or right ventricles demonstrated that different groups of ganglion cells projected to discrete or selective regions of the heart. Projections to the left ventricle originate only from ganglion cells located posterior to the interatrial septum and the left atrium. In the rat, intracardiac ganglion cells, confined to 4 atrial regions, appear to have discrete sites of termination within the heart. It is proposed that selective activation of different intracardiac ganglion cell groups may elicit specific regional changes in cardiac parasympathetic nerve activity.