Cardiac output is modulated by the autonomic nervous system (ANS driving the intracardiac nervous system (ICNS), the final common pathway for cardiac control. Neural regulation of the 4-chambered mammalian heart is complex, and because the ICNS is distributed throughout both atria and cannot be accessed in its entirety for study, our understanding of neural control of the mammalian heart is incomplete. With two chambers rather than the four of the mammalian heart, the hearts of zebrafish offer a more accessible experimental model for analyzing the neural control of cardiac output. Similar to the mammalian heart, fish hearts are dually innervated by the sympathetic and parasympathetic limbs of the ANS; the overall functions of these limbs also similar to their functions in mammalian hearts. In this study we aim to refine and elaborate upon previously established anatomy through identification of neural pathways in the zebrafish and the myocardial targets of autonomic innervation. In combination with immunohistochemical methods, electrophysiology and optical imaging will elucidate the types of responses evoked in the ICNS by vagal stimulation and the contribution of each limb of the ANS. It will thus be possible to eventually identify all the components of specific functional pathways that control heart rate or myocardial contractility, identification as yet not possible in the mammalian heart.