BACKGROUND Successful prediction of reentrant ventricular tachycardia duration and form from sinus-rhythm electrogram signals in canine hearts is relevant to clinical studies, to potentially improve catheter ablation treatment during EP study. RESULTS Following LAD ligation of canine hearts, activation maps were constructed from 312 border zone sites 4-5 days postinfarction. When reentrant ventricular tachycardia was inducible via programmed stimulation, the core of the circuit was defined on the maps as the enclosed area formed by the adjoining lines of slowest conduction and block bounding the protected region of the reentrant circuit. The number, location, and width of points of entrance or exit of the activation wavefront about the core were determined. Core perimeter location was then marked on the sinus-rhythm activation map, and the difference in activation time at opposite recording sites along the core perimeter was measured. Mean sinus-rhythm activation was highly discontinuous along the core perimeter in 10 transient reentry experiments (30.1 +/- 4.4 ms), moderately discontinuous in 13 sustained experiments (16.7 +/- 1.8 ms), and only slightly discontinuous in 5 noninducible experiments (9.7 +/- 1.7 ms). For transient versus sustained experiments, the entrance/exit points were narrower (mean: 6.5 +/- 1.0 mm vs 9.5 +/- 1.8 mm) with larger sinus-rhythm discontinuity across them (mean: 23.8 +/- 6.0 ms vs 11.8 +/- 2.1 ms). As core size increased, so did the number of entrance/exits present during reentry (P < 0.001). With increasing core size, four-loop (quatrefoil) reentry was frequently observed. CONCLUSIONS Whether reentrant ventricular tachycardia will be inducible in the canine infarct border zone, and its duration and form, is associated with the characteristics of electrical discontinuities present about the core perimeter.