Background: Elucidation of the electrophysiological mechanisms of nonsustained ventricular tachycardia (VT) in humans is required to define the relationship between nonsustained VT and sustained VT. This goal requires, at least in part, analysis of transmural ventricular activation in patients with both sustained and nonsustained VTs. Methods and Results: We analyzed three- dimensional intraoperative cardiac maps of extrastimuli and beats during 44 nonsustained VTs and the initiating beats of 6 sustained VTs from six patients with healed myocardial infarcts who were undergoing arrhythmia surgery. The coupling interval, total activation time, and diastolic interval of each extrastimulus and boat of nonsustained VT were compared with counterparts during sustained VT. Sites activated last during extrastimuli initiating nonsustained or sustained VTs occurred in the same region, and activation times were comparable. However, the site of earliest activation during the initial or subsequent beats of nonsustained VT was discordant from the site activated earliest during the first and subsequent beats of sustained VT in 74% of cases. The mean variance in coupling interval, but not total activation time or diastolic interval, was significantly greater for VT that terminated before the 10th cycle than for VT that sustained. When analyzed from the last extrastimulus up to the fifth VT cycle, the standard deviation of the coupling interval, but not of the total activation time, was greater for nonsustained than for sustained VTs. Electrode density was sufficient to define an arrhythmia mechanism for 36 beats of nonsustained VT. Twenty-one (58%) initiated in the subendocardium, midmyocardium, or epicardium by a macroreentrant mechanism, and 15 (42%) initiated in the subendocardium by a focal mechanism. Conclusions: Compared with sustained VT, nonsustained VT initiates at discordant sites, is characterized by oscillations in coupling interval but not in total activation time, and initiates by either a macroreentrant or a local mechanism.