Arrhythmias are common in the natural history as well as in the late postoperative course of patients with atrial septal defect (ASD); electrophysiologic disturbances may result from the ASD itself or from surgery. Electrophysiologic studies were performed in 18 children (mean age 10 years) both before and after surgical closure of the ASD. Sinus node (SN) function, conduction intervals and refractory periods (atrial and atrioventricular [AV] nodal) were determined. Before surgery, corrected SN recovery time was prolonged in 14 patients; the mean value for the group was 357 +/- 163 ms. The AH interval was slightly prolonged, as were AV nodal refractory periods. Postoperatively, SN recovery time decreased in all patients who remained in sinus rhythm (p less than 0.02), but 5 patients had atrial ectopic rhythm. The AH interval decreased significantly (p less than 0.02), as did the refractory periods, mainly for the AV node (p less than 0.01). The pacing rate at which second-degree AV block occurred increased. Thus, closure of ASD improves AV conduction, decreases AV nodal refractory periods and improves SN function, probably by suppressing rightsided heart volume overload. However, SN function may be lost, probably as a result of the operative procedure.