A recently developed optical method utilizes a single, implantable, optical fiber to record electrical activity from isolated hearts stained with voltage-sensitive dyes. This optical technique generates recordings of transmembrane potential from excitable myocardial tissue, and remain free from stimulus artifacts that accompany electro stimulation and hinder all standard electrode recording methods during the application of high-voltage electrical shocks. The fiber optic system uses a l00J,lm diameter core fiber which can record from epicardial surface, internal mid-myocardium, or endocardial surfaces. The stained tissue is excited through the fiber and the resulting fluorescence is transmitted through the same fiber to a photomultiplier tube. Changes in fluorescence accompanying normal cardiac action potentials usually range from 1-5%. Substantive motion related signals accompany normal beating hearts, drowning out the actual signal that corresponds to change in membrane potential. When added to the nutrient solution of the heart, an excitation / contraction decoupler restricts motion and reduces the motion related signal making it easier to isolate the true membrane potential signal.