The investigation of the influence of microstructures in cardiac conduction disturbances requires a detailed analysis of the propagating process at a microscopic size scale. We present a system for recording extracellular potentials to measure the instant of the local depolarization with very high-resolution ( 3 0~s in time and 15 p m in space ). Micromaps of the extracellular activation indicated discontinuous conduction at a microscopic size scale. Time series of subsequent activation patterns showed an extraordinary small variability of all conduction times (STD < 20ps) under normal conditions. These microfluctuations are influenced by arrhythmogenic factors, antiarrhythmic drugs and uncoupling substances. INTRODUCTIONThe measurement of the excitation spread in cardiac muscle at a microscopic level is becoming of increasing practical importance. Recent experimental and theoretical findings confirm the hypothesis of the discontinuous nature of the propagation in heart tissue corresponding to the complex microstructure [1] [2]. To monitor micropatterns of excitation, measurements of high-resolution in time and space without injury of cells are required. We have developed a recording system to measure extracellular action potentials with the required resolution in time and space. With the described experimental setup sequences of the extracellular action potentials were analyzed and beat-to-beat microfluctuations of conduction velocities were studied.