Abstract: Objective To establish the background database of spontaneous tumors in F344 rats bred at the National Beijing Center for Drug Safety Evaluation and Research. Methods Three-hundreds and thirty six F344 rats weighing 180-200 g, maintained in SPF condition were included in this study. They were routinely bred at the barrier facility of the center. After 6, 12, 18 or 24 months, the rats were euthanized at the number of 32, 64, 64 and 176, respectively. The rats were grossly examined and the weight of heart, lung, liver, spleen, kidney, adrenal gland, thymus, brain, testes (ovaries for the female) and epididymis (uterus for the female) were recorded and the coefficients of the organs were calculated. Tissues were routinely processed for paraffin sectioning and H.E. staining and histopathological examination were performed. The spontaneous tumors were classified and their incidence rates were calculated. Results The frequently found neoplasm in F344 rats include leydig cell tumor, adenoma in pituitary gland, large granular lymphoma (LGL), subcutaneous fibroma, mammary adenoma and pheochromocytoma. Conclusion The present study provides the background database of the spontaneous tumors in the F344 rats, and may be laid a good foundation for the carcinogenesis studies for the new drugs.

Key words: Rat, Carcinoma, Pathology