Immediate and long-term effects of a transverse circular incision of the entire right atrium, including the atrial septum, on internodal conduction were studied in 7 dogs. Electrocardiograms were followed up to 35 weeks, and the atrial epicardial excitation sequence was evaluated in all 7 dogs. The findings were compared with data from 3 normal dogs and 3 other dogs which underwent total atrial transverse incisions. All dogs with total atrial separation developed complete atrioventricular (A-V) block immediately after the operation and died within 3 days. All dogs with right atrial interruption exhibited persistent sinus rhythm with slight extension of P-R intervals and a normal QRS immediately after the operation. The mean P-R interval reached its maximum at the fourth day (50 per cent increase), remained prolonged through the first week, but returned to near control valves after 12 weeks. Three days later developed transient arrhythmia; sinus arrhythmia; sinus arrhythmia in 2 dogs and second-degree block in 1 appeared between the tenth and fourteenth postoperative days. Postoperative P vectors migrated toward the left. Atrial excitation sequence studies demonstrated prolongation of atrial epicardial conduction intervals (2.3 times) between the sinus node and distal right atrial wall. The presence of an interatrial conduction mechanism (whether by specific pathways or not), through which sinus node excitation was transmitted to the A-V node was demonstrated.