Recovery cycles following premature atrial stimulation introduced during atrial pacing may fall into four categories: reset, incomplete interpolation, complete interpolation, and echo re-sponses. It has been postulated that the transition from reset to incomplete interpolation may represent the point at which premature beats can no longer enter the sinus node and reset it and thus reflect the refractory period of the sinus node. The purpose of this study was to explore the electrophysiological basis underlying the transition from reset to incomplete interpolation and. to assess the spatial orientation of refractoriness in the sinus node. In 15 rabbit sinus node preparations, premature atrial stimuli were introduced at varying degrees of prematurity while intracellular potentials were recorded with a microelectrode in the sinus node. In 12 of 15 experiments, transition from reset to incomplete interpolation immediately followed a sudden reduction in action potential amplitude, rendering the action potential incapable of resetting the node. This point was interpreted as the effective refractory period of the sinus node. During the zone of incomplete interpolation, low voltage depolarizations were seen in the node, interfering with diastolic depolarization and delaying the recovery beat. These small depolarizations were absent during the zone of complete interpolation. In six experiments, the micro-electrode was moved toward the crista terminalis in steps of 50 to 100 /im, and the stimulation sequence at each site was repeated. By examining relative action potential amplitude at various sites at the same premature interval, it was possible for us to construct curves showing the pattern of block of premature impulses. We found that progressively earlier premature beats are blocked at progressively greater distances from the node. Therefore, tissue between the crista terminalis and the sinus node provides a progressive gradation of refractoriness, rather than a discrete barrier.Circ Res 47: 742-756, 1980