It is generally believed that the atrioventricular node (AVN) possesses filtering properties (FP) capable of preventing excess atrial impulses or earlier atrial premature beat from reaching the ventricle. In the present study, the refractory periods (RP) of cardiac tissues and the FP of the AVN were investigated in isolated rabbit hearts (n = 18). It was found that: (1) The relative RP of AVN was the longest at shorter basic cycle lengths (BCL) (200-300 ms) while the relative RP of His-Purkinje system (HPS) was the longest at longer BCL (600, 700 ms). (2) The effective RP of AVN was shorter than the functional RP of the atrium in most cases (16/18) at any BCL. (3) During rapid atrial pacing at cycle length of 140 ms in 7 cases, AVN maintained 1:1 conduction. (4) The atrioventricular nodal conduction curve was smooth at BCL from 200 to 700 ms. The present results suggest that AVN in the isolated perfused rabbit heart do not have the FP and any dual atrioventricular pathways in a physiological sense.