A system is described that uses a scanned laser beam to excite voltage-dependent fluorescence in cardiac muscle and thereby monitors propagation of the action potential. Details of the optical and electronic design are presented along with descriptions of the system performance. The scanner can monitor membrane voltage activity from 64 points simultaneously at a sample rate of 1000 samples/sec. Results are presented from a laser scan of arrhythmic guinea pig left atrium showing the complete evolution, from initiation to termination, of a functional microreentry induced by a premature stimulus.