It is difficult to estimate the depth of the focal source by activation mapping. The present study was performed to demonstrate the usefulness of a simple maneuver in estimating the depth of the focal activation source (S). A total of 44 sites (15 shallow, depth<3.5 mm; 29 deep, depth>5.5 mm) were analyzed in 3 canine left ventricles under general anesthesia. A custom-made bipolar needle electrode was used to simulate a focal activation source. A mapping catheter with an electrode tip size of 2 mm, band electrode size of 1 mm, and inter-electrode spacing of 2-10-2 mm was placed at the mapping area. The position of the center of the distal 2 electrodes was kept at the insertion site of the needle electrode. The time interval between distal and proximal electrodes of the mapping catheter (Ttachy) was measured during needle electrode pacing. The time interval between distal and proximal electrodes (Tpace) was measured during pacing with distal electrodes of a mapping catheter. Depth index (δ) was defined as Ttachy/Tpace. Using in vivo data, simulation was performed to evaluate the depth and δ. Ttachy was 24±7.7 ms and 15±5.6 ms for shallow source and deep source simulation, respectively (p<0.001). δ values were 0.86±0.21 and 0.55±0.21 for shallow source and deep source simulation, respectively (p<0.001). According to simulation data, if δ<0.52, the depth of the focal source will be >5.5 mm. Ttachy was shorter and δ was smaller for a deep S than for a shallow S. Copyright © 2020. The Korean Society of Cardiology.