It is often difficult to differentiatepremature ventricular contractions(PVCs) frompremature atrial contractions(PACs) with aberrant conduction by means of surface electrocardiograms. This differentiation is of considerable importance, as the treatment for the two entities differs. Premature beats commonly occur following open-heart surgery. Often during open-heart surgery, the atrial and ventricular pacing wires are inserted and brought to the surface. It is then possible to record the epicardial atrial and ventricular electrograms from these pacing wires. In this paper, the differentiation between PVC and PAC with aberrant conduction is performed using a microcomputer. To minimize the system's cost, the Radio Shack TRS-80 Color Computer, a commercially available personal microcomputer, was chosen as the basis for the monitoring system. After filtering and amplification, the ECG is digitized and analyzed. Once atrial or ventricular activity has been established, an algorithm is used to determine whether the heartbeat was a normal beat, a PAC, or a PVC. This algorithm is based upon the fact that normal beats, PACs, and PVCs have the P wave and QRS complex in a known sequence, with an identifiable time between beats. The occurances of PACs and PVCs are monitored and recorded for future analysis.