To understand the vectorcardiographic (VCG) patterns in rats, we studied 50 adult male Long-Evans rats weighing 250-350g. Under anesthesia with pentothal, the rat was fixed to woodplate in a supine position with extremities extended. Surface electrocardiograms (ECG) and VCG of Frank lead system were then recorded in all rats. All variables of ECG and VCG were measured and compared with those of humans to determine the main difference. We found that rats have prominent anterior force of QRS complex and large ST vector in both ECG and VCG as compared to those of humans. The prominent anterior force of QRS complex may be due to different body shape between rat and human chests, and the large ST vector is probably related to early ventricular repolarization in rats. These findings should be considered in any experimental condition involving the ECG and VCG in rats.