On vectorcardiogram in Wistar Kyoto rats, the relationship between maximum spatial QRS vector determined accurately by VCG analyser(VAC-3) and the index of ventricular mass was analysed. In the Frank and McFee Parungao lead systems there were no significant correlations between the magnitude of maximum spatial QRS vector and the measurements of heart and ventricles. These results suggest the necessity to modify the Frank and McFee Parungao lead systems, or to produce a new lead system for rats.