The first ten cycles following cessation of atrial pacing were evaluated in 44 control subjects (mean age 52.9 +/- 14.88 yr) and 39 patients (mean age 62.9 +/- 15.41 yr) suspected of having sinus node dysfunction (SND). The maximal cycle length for each postpacing cycle following several pacing periods in each control subject was determined, and was normalized by dividing it by the subject's mean spontaneous control cycle length (SCL). Using the control group, a normalized maximal post-pacing response pattern (mean and SD) was derived. For each SND patient, a composite SD for each post-pacing cycle was calculated by adding the patient's SCL variance to the variance determined for each post-pacing cycle in the control group. Two composite SD above the mean value for each post-pacing cycle was selected as the upper limit of the normal recovery response and used to identify abnormal post-pacing responses in the 39 SND patients. Abnormally prolonged cycle lengths subsequent to the first escape cycle (secondary pauses) were found in 16/39 (41.0%) patients, of whom 11/39 (28.2%) had a prolonged SNRTmax. Of importance, 11/12 (91;7%) patients with documented SA block or sinus pauses prior to electrophysiologic study, demonstrated secondary pauses, while only 7/12 (58.3%) had a prolonged SNRTmax. Criteria are derived for the identification of secondary pauses during the postpacing period, and a close association between secondary pauses and the presence of spontaneous SA block or sinus pauses prior to electrophysiologic study is demonstrated.