A system for performing real-time analysis of cardiac action potentials has been developed using a microcomputer based on the Motorola 6800 central processor. Transmembrane potentials obtained using standard microelectrodes were digitized to 8 bits at 80 ms intervals, stored in cyclic memory buffer from which they could be selected for analysis, converted back to analogue form and displayed on an oscilloscope in real time. For each action potential, amplitude, the maximum rate of change of potential and the action potential duration at 50% repolarization and 100% repolarization were measured. In addition, conduction time (taken as the interval between the stimulus artefact and the action potential) was measured. All data were stored in memory and later printed, together with the time at which the recording was made. The system was designed for analysis of action potentials recorded using floating microelectrodes. The computer was controlled by three remote switches and a potentiometer, positioned close to the muscle bath. The program was written in Motorola 6800 assembly language and stored in erasable programmable read-only memory.