Background: According to the spiral wave hypothesis of re-entry, the core of functional reentry remains excitable but not excited. We sought to determine whether the core remains excitable and whether excitation of the core by an outside wave from leads to termination of the reentry in the atrium. Methods and Results: In nine isolated canine right endocardial atrial tissues (3.8 by 3.2 cm wide), reentry was induced by a premature point stimulus (S2). The isochronal activation maps and dynamics of the activation patterns were visualized with the use of 509 bipolar electrodes (1.6-mm resolution). The S2 applied after 8 regular beats induced reentry with a mean cycle length of 162±20 ms (15 episodes). Reentry had a large excitable gap (93±26 ms) as determined by early capture with twice the level of threshold stimuli. The central area (core) around which the wave fronts rotated had a mean surface area of 12±3 mm2. The electrograms located in the core of the reentry registered no or very low amplitude potentials. In 13 of 15 episodes, reentry terminated when an outside new wave front merged with the original wave front and excited the core. Core excitation caused disruption of the original wave front, and the newly formed wave front(s) vanished at the tissue border within 77±18 ms. In 2 episodes, reentry terminated abruptly when an outside new wave front propagating in a direction opposite to the reentrant wave front collided with the leading edge of the reentrant wave front. Conclusions: Functional reentry in the atrium is compatible with a spiral wave of excitation with an excitable but nonexcited core and a large excitable gap. Reentry may be terminated either by direct excitation of the core that displaces the wave from to the tissue border or by collision with an outside new wave front.