For monitoring the qualitative change of Wistar-Imamichi Rat introduced from Japan in 1986, the curves of body-weight and the stability of 4-day sex cycle have been tested continuosly from age of 4 weeks to 10 weeks in 20 females and 6 weeks to 12 weeks in 5 males. The results Showed that there was no significant difference in average weekly increasing body-weight between introduced rats at our Research Center and the original Wistar-Iamichi Rats from Japan ( P>0.05).Stable 4-day sex cycle of the females emerged at age of 39-55 days in introduced rats, 70% ( 14/20 ) of them were distributed in the age of 40~50 days, and 93.3% (14/15) of Wistar-Imamichi Rats from Japan in the age of 40~50 days. There was no significant difference between them (P>0.05) .