In order to determine whether the A-H interval of the His bundle electrogram accurately represents the AV nodal conduction time under various conditions, His bundle and coronary sinus electrograms were recorded in isolated perfused rabbit hearts, with atrial stimulation from eight different sites. The S-A (stimulus to the A wave) interval was significantly longer, whereas the A-H interval was shorter on stimulations from the coronary sinus and the left atrium than on sinus nodal stimulation. Stimulations from the fossa ovalis and right atrial appendage did not significantly alter the A-H interval. The effective and functional refractory periods of the AV node were almost identical with stimulations from the sinus node, low right atrial appendage, low left atrial appendage or the ostium of coronary sinus. Mapping of the excitation process with microelectrodes revealed that the activation times in low interatrial septal fibers bordering the His bundle [abbreviated as AP(III)] was closest to the A wave as compared with the activation in the ostium of the coronary sinus [AP(I)] or near the AN region [AP(II)] on stimulation from both sinus nodal and coronary sinus regions. On sinus nodal stimulation, AP(III) preceded AP(I) and AP(II) but lagged behind the A wave by 6 msec, whereas AP(III) preceded the A wave by 5 msec on coronary sinus stimulation. Thus, the A-H interval may not always accurately represent the intranodal conduction time, as relative timing of atrial activation responsible for the A wave and that of invasion of the AV node by the atrial wavefront can be grossly altered by different atrial excitation patterns.