Traditional analyses have assumed that cardiac electrical activity is reflected on the surface electrocardiogram (ECG) without distortion as the signal passes through the body tissues. This study aims to explore the frequency-dependency of thoracic attenuation of surface-recorded intra-cardiac electrical activity. Twenty patients (14 males, mean age 55±15 years) undergoing electrophysiologic study were enrolled. Rectangular unipolar stimuli were applied from a catheter positioned in the right ventricular apical area and another in the posteroseptal area without contact with the myocardium. An orthogonal Frank-lead surface ECG and a unipolar intra-cardiac electrogram near the pacing site were recorded. Frequency domain characteristics of the signal-averaged pacing impulses were analyzed. Linear regression analysis showed significant frequency dependent attenuation in the magnitude transfer functions (R² ranged from 0.84 to 0.89, p<0.0001) and good linear fit for the phase transfer characteristics (R² ranged from 0.98 to 1.0, p<0.0001). Age, physical dimension, and respiratory characteristics had significant effects on both the magnitude and phase characteristics of the transfer functions. Applying models of the low slope and high slope transfer functions to signal averaged ECGs from 33 subjects showed differences in the attenuation of P-waves and T-waves relative to the QRS.