Background: One problem associated with end to end anastomoses after preoperative radiotherapy is leakage. The strength of the anastomoses is very important. Fractionation schedules are widely under investigation to achieve higher doses with less toxicity and high curative potential. The aim of this study was to compare preoperative conventional and hyperfractionated radiotherapy effects on anastomoses. Materials and Methods: Sixty Wistar albino rats were divided into 3 groups: control, conventional radiotherapy (2 Gy/fraction, total 44 Gy), and hyperfractionated radiotherapy (1.2 Gy/fraction, total 52.8 Gy). 5 weeks after completion of the treatment, the rats underwent end to end anastomoses to the rectum. Following the surgery, the bursting pressure of the anastomoses and the hydroxyproline levels were measured on days 3 and 7. Results: The bursting pressure was lower in both radiotherapy groups on the 3rd day. On day 7, the results of the conventional radiotherapy arm matched those of the control group, but the results of the hyperfractionated group did not (p = 0.001). On day 3, hydroxyproline levels were significantly lower in both radiotherapy groups than in the control group (p = 0.001). On the 7th day, the statistical difference disappeared within the radiotherapy groups. However, the hydroxyproline levels of the 2 radiotherapy arms were still lower than those of the control group (p = 0.001). Conclusion: Hyperfractionation strongly inhibits collagen synthesis and is associated with protracted healing.