Abstract: The incidence of spontaneously occurring tumors in the selected TR 1 and TR 2 inbred strain rats was observed over two years and reported the results of histological examinations as follow. The TR l rats used were 288 （□105；♀ 183） Mammary gla-nd fibroadenomas in the TR 1 rats were 9.3% （♀）and 1.0% （□）,mammary gland adenocarcinomas 1.6% （♀）,lung adenomas 1.6% （♀） and 2.9% （□）. The TR 2 rats used were 336 （□186；♀150）.Mammary gland fibroadenomas were 13.3%（♀） and 2.2%（□）, mammary gland adenocarcinomas 2.7% （♀） and 0.5% （□）, lung adenomas 0.7% （♀） and hepatomas 0.3% （1/336）。