Electrical cardiac mapping has been used to study the mechanisms of cardiac arrhythmias, to assist in clinical diagnosis of rhythm disorders, to guide interventional procedures, or to evaluate the effects of antiarrhythmic drugs. Manual determination of local activation times, the first step in constructing activation maps, is a time-consuming process that precludes the possibility of on-line interactive mapping during an electrophysiologic experiment or a clinical procedure. This report describes an interactive graphic user interface application that (1) automatically determines the activation sequences with good accuracy, (2) displays graphical presentations of activation maps within seconds, and (3) allows manual interactive adjustments. Five automated activation time detection algorithms, one for bipolar electrograms and four for unipolar electrograms, were evaluated and compared. High-density canine recordings were used to evaluate the accuracy of the system. Data included normal atrial activation (sinus rhythm) and abnormal reentrant atrial activation (atrial flutter).