The Nehb lead system was examined to emphasize and find out at early stage ST depression of the left precordial leads (V(5), V(6), V(7)). The results were as follows. 1) The 19 patiens with ischemic heart disease were recorded after exercise tests. The mean QRS vector was situated on the left, posterior and inferior. The mean ST vector was directed to the right, anterior and superior. Namely, the former tended to be at a 180° angle to the latter. 2) The experiments of a torso, which was made by atrunk model of a japanese adult man, proved that a magnitude of the lead Nehb-D was larger than those of the leads V(5), V(6), V(7). From the image surface which was obtained from the experiments of the torso the direction of the lead Nehb-D was situated on the same left, posterior and inferior as the leads V(6), V(7), especially close at the lead V(7). The lead Nehb-A was the most largest magnitude, but the direction was situated on the same left, anterior and inferior as the V(5). The directions of the leads Nehb-D, A, J and the leads V(5), V(6), V(7) were the same in both azimuth and elevation and the image surface. Accordingly, the lead Nehb-D is usefull to emphasize and find out ST depression earlier than in the left precordial leads (V(5), V(6), V(7)).