A balloon array of 112 electrodes was used to obtain simultaneous recordings of endocardial electrograms during intraoperative mapping studies of ventricular tachycardia. Introduction of the balloon through a left atriotomy and across the mitral valve allowed endocardial activation maps to be obtained in the intact left ventricle. Of 20 patients with coronary artery disease studied in this way, suggestive evidence of endocardial reentry was found in 6. Three separate reentrant mechanisms were observed. In two patients, a single broad wave front of continuous recirculating activation reminiscent of a vortex was initiated by the formation of a functional arc of block in response to premature stimuli. In five patients, premature stimuli again produced a functional arc of block, which was circumvented by two opposing wave fronts that united on the distal side. Retrograde penetration by a narrow isthmus of slow conduction through the block initiated the tachycardia, whose activation sequence was consistent with figure eight reentry. In one patient, premature stimuli produced a region of delayed potentials. Critical timing of these resulted in microreentry in an adjacent circumscribed site, which formed the site of origin of the ensuing tachycardia. The microreentrant signals were not detected by standard unipolar recordings, but were seen on simultaneously recorded high gain electrograms. In 14 patients, although mapping identified a site of origin, the activation patterns showed either radial spread or incomplete circles. Detection of reentrant mechanisms during intraoperative mapping required high density electrode arrays and refined high gain recordings. An intact ventricle may facilitate intraoperative initiation of tachycardia.