Summary:His bundle electrograms were recorded in 8 rats by a signal averaging technique using the Takayasu vectorial lead system. Animals were anesthetized and placed in the prone position. The polarity of three orthogonal leads were: the X axis, from right to left; the Y axis, from top to bottom; the Z axis, from back to front. Potentials from the X, Y, and Z leads were amplified by 20,000 with high-pass 12 dB/octave filtering at 80 Hz, and signal averaging of 2000 beats was performed at a sampling interval of 100 ps. The His bundle potential could be clearly defined in all 8 rats. The mean amplitude of the His potential was larger in the X-axis (23.6f9.2pV) or the Y-axis (28.4k 14.5pV) than the Zaxis lead (1 1.5 f 8.6pV). Directions of the His potential vectors were to the left in 5 of 8 rats (62.5%), to the caudal in 4 of 6 rats (66.7%), and to the dorsal site in 6 of 8 rats (75.0%). This vectorial lead system, devised in accordance with McFee and Johnston's theory on lead field, was useful for recording His bundle electrograms.