Spinal afferents relay sensory information such as visceral pain to the central nervous system. Yet, the morphology and distribution of spinal afferent axon terminals in the whole heart are not fully delineated. Factors for this include difficulty of surgery, tissue thickness, and viable tracer options. To surmount these difficulties without the use of sectioning, which disrupts the axon’s structural integrity, we used male Sprague-Dawley rats and injected the tracer dextran biotin (DB) into the left DRG (C8-T3). After 16 days, the left & right atria and ventricles were harvested then further steps of tracing, digitization, and imaging were taken. Our findings are: In the atria, DB-labeled axons entered from the left precaval vein (LPCV) then projected to both atria with a higher concentration in the left atria. Innervation was located in the epicardial, myocardial, and endocardial layers. Target of atrial spinal afferents included cardiac muscle, vasculature, intrinsic cardiac ganglia (ICG), and adipose tissue. In the ventricles, spinal afferents projected into the left and right ventricles from their bases. They distributed throughout both ventricles, with their highest density being in the left ventricle. DB-labeled axons were identified in the epicardial and myocardial layers. Spinal afferents formed both simple and complex terminal morphologies and made close contacts with adipose tissue, cardiac muscle, and blood vessels. This work successfully labeled, traced, and digitized the spinal cardiac afferents in the flat-mounts of the whole heart and further marks the first comprehensive characterization of spinal afferent axon distribution and morphology in whole ventricle flat-mount preparations of the rat. This research will provide a foundation for specific labeling of cardiac spinal afferent fibers. In addition, it will lead to the future study of topographical cardiac spinal afferent innervation, functional studies, and their remodeling in pathological conditions.