To determine whether the sinus node electrogram (SNE) could be detected noninvasively, a specially modified P wave-triggered signal-averaged electrocardiogram was recorded through the standard V1, V2 and V3 precordial leads in 39 patients. The data between 500 ms before and 300 ms after the trigger point were processed using a digital bandpass filter of 0.5 Hz (12 dB/oct)-20 Hz (18 dB/oct). Pre-P signals were observed just before the P wave resembling SNE recorded directly. Pre-P potential duration closely correlated with direct SACT (r = 0.856, P < 0.0005). Thus, it is believed that signals obtained by this new method would be related to sinus electrical activity and that it could be useful for noninvasive estimation of SACT.