Objective: This experimental study aims to investigate whether radiotherapy (RT) plus trastuzumab (T) followed by subsequent hormonotherapy increase the cumulative toxic effect on cardiac functions in rats. Methods: A total of 70 Wistar-Albino female rats with a mean weight 213±27 g were randomly divided into equal seven groups. The first group (C) underwent no procedure. The second group (RT) underwent the whole thoracic radiation including heart. The third group (T) was administered T through tail vein alone. The fourth group (RT+T+Tx) was administered T initially and the whole thoracic radiation at two hours, followed by tamoxifen at one week. The fifth group (RT+T+Le) was administered T and then underwent thoracic radiation, followed by letrozole. The sixth group (T+RT+An) was administered T and then underwent thoracic radiation, followed by anastrazole. The seventh group (T+RT+Ex) was administered T and then underwent thoracic radiation, followed by exemestane at one week. Hormonotherapy was administered to the rats in the Group 5, 6 and 7, as indicated in the Group 4. Radiation therapy was administered following T treatment at two hours as a single 12 Gy fraction. After the rats were sedated under anesthesia and sacrificed at 24 weeks, cardiac tissues were removed. Serial sections obtained following paraffin blockage were stained and the ratio of myocardial fibrosis was assessed. According to statistical analyses by the one-way ANOVA test and Tukey HSD test, a significant difference between test components. Results: At the end of the study, no loss and adverse effects were seen in any group. There was a statistically significant difference among atrium, ventricle and aorta (p<0.001). The mean value of fibrosis scores increased in the rats which underwent RT. In the assessment of atrium, a significant difference was found between RT group and RT+T+An group and also T group and RT+T+Fe group (p<0.001). In the assessment of ventricule, a statistically significant difference was observed among RT, RT+T+An and RT+T+Ex. In the assessment of aorta, the scores of RT group was significantly higher than RT+T+An and RT+T+Ex. A statistically significant difference was observed among these groups (p<0.001). Conclusion: Our study results suggested that there was no significant additional cardiotoxicity of adjuvant hormonotherapy following concomitant RT and T treatment, compared to RT in terms of cardiac fibrosis.