The AV node is a structure in the atrial septum at the atrioventricular junction. The atrial septum is important for various types of supravenricular arrgythmia especially for AV node reentrant tachycardia. In this study, it was aimed to understand the electrical propagation in the atrium and the atrial septum, as well as the effects of perinodal cryosurgery. Eleven mongrel dogs weighing between 26.0kg and 34.5kg were divided into control(d dogs) and cryosurgery(6 dogs) group. The animals were anesthetized with pentobarbital sodium(30 mg/kg) and under a normothermic total cardiopulmonary bypass, endocardial template electrodes(left atrium ; 124, right atrium; 128 bipolar electrodes) were placed into the both atria. The endocardial electrocal activation was recorded using a 256 channel computerized mapping system during normal sinus rhythm, atrial pacing, and ventricular pacing. For the cryosurgery group, the endocardial electrical activation was recorded after eight weeks of discrete perinodal cryosurgery. The results are as followings : 1) The electrical propagation from the SA node to the AV node occurred mainly through thick muscle band of the crista terminalis of the right atrium and anterior limbus fossa ovalis. 2) Electrophysiologically, the atrial septum appeared to be 2 layers. 3) During ventricular pacing, the center of the Koch triangle was the first to respond to activation. When the atrial septum of the AV nodal area was activated, the electrical activations propagated asymmetrically and the activity of the left atrium was propagated through the Bachmann's bundle. The left atrial side of the atrial septum was activated 10-15 msec later than the right side during ventricular pacing. 4) The cryosurgery did not change any significant difference in the AH, HV, AV node effective refractory period and AV node Wenckebach period. However, the ventriculo-atrial conduction was blocked in 50% of the animals. 5) In cryosurgery group, the electrical propagation from the SA nodal area to both atrial was similar to the control group except along side the vertical right atrial incision site. 6) The endocardial activation map in the cryosurgery group showed that the activation was spread out from the lateral atrial tissue outside of the cryolesions. The left atrial side of the atrial septum was was activated 6-19 msec later than the right atrial side. In conclusion, electrophysiologically the atrial septum appears to be composed of 2 layers and there is no direct electrical propagation between the AV node and the left atrial side of the atrial septum. The cryosurgery did not influence the electrial activation sequence from the SA node to the AV node except the site of cryolesion.