The structure and function of the atrioventricular junction (AVJ) has been extensively studied for a century in many species. Historically, studies of the human AVJ were limited to postmortem histologic investigations because functional studies were not possible. With the advent of catheter based approaches, the function of the human AVJ has been studied and catheter based treatments have been developed. However catheter based approaches are limited both in the number of recording sites and the interventions possible to investigate AV conduction. In animal studies, optical mapping has emerged as the best technique to record electrophysiologic properties of nodal tissues, however this technique has not yet been applied to human AV conduction. In this study, we present the first instance of optical mapping of atrioventricular conduction in the human. Explanted human hearts were obtained at the time of cardiac transplant from the transplant team at Washington University. The heart was removed from the body, perfused with cardioplegia, and transported to the laboratory. The AV nodal artery was cannulated and optical mapping was performed with a 16×16 photodiode array and the voltage sensitive dye Di-4-ANEPPS, in the presence of the excitation-contraction uncoupler blebbistatin (5uM) [1]. Optical action potentials (OAPs) were recorded from the interatrial septum, His bundle, AV node, slow pathway, and ventricular septum of human triangle of Koch preparations (n=2). S1S2 protocols repeatedly induced slow-fast reentrant beats: the slow pathway conducted anterogradely, followed by retrograde excitation of the fast pathway and subsequent excitation of the slow pathway again. When the slow pathway conducted to the His, the His electrogram had a different morphology than when the fast pathway was responsible for His excitation, a phenomenon which indicates differential excitation of the His by the fast and slow pathways [2]. Additionally, a junctional rhythm was recorded from the His bundle region which demonstrated diastolic depolarization in the His OAP. The preparations that were functionally characterized will be studied with histology and immunohistochemistry to determine the precise structures and molecular characteristics responsible for OAPs. This study is the first demonstration of optical mapping of conduction in the human AVJ during pacing, spontaneous activation, and AV nodal reentry. Our preliminary results reveal a pathway of AV nodal reentry, and our results indicate that optical mapping can be implemented to investigate the characteristics and mechanisms of human AV conduction in vitro.