【Abstract】Objective To investigate the significance of cyclin D1 and p53 protein expression in synchronous breast carcinoma and fibrosarcoma in rats. Methods Immunohistochemical SP methods was used to study the expression of cyclin D1 and p53 in synchronous breast carcinoma and fibrosarcoma induced by DMBA in rats.Results There was no expression of cyclin D1 and p53 in normal breast tissue. In atypical hyperplasia of mammary, there was overexpression of cyclin D1(7/14) and no expression of p53. The overexpression of cyclin D1 and p53 were detected in breast carcinoma (8/18,7/18 respectively) and fibrosarcoma (9/14,5/14 respectively). There was no expression of cyclin D1 and p53 in adjacent sarcoma.The expression of cyclin D1 and p53 protein was associated with histological grading, and showed inverse relation between them. Conclusion There are cyclin D1 and p53 protein overexpression in the synchronous breast carcinoma and fibrosarcoma induced by DMBA in rats. Cyclin D1 may paticipate in the course of the carcinogenesis of breast carcinoma and fibrosarcoma in rats, and p53 protein overexpression may relate to the degree of malignancy of the tumors.