A comparative study was undertaken to evaluate the possible effects of time selection of irradiation on the healing of microvascular anastomoses. In this experimental study, 40 adult male Sprague-Dawley rats were divided into four groups, 10 animals in each. In Group 1, the rats were exposed to radiotherapy with a dose of 20 Gy in a single fraction at a 170,916 cby/min dose rate, at an 80-cm source to a skin distance from 3 to 4 cm in the right groin Fields involving the femoral artery, using a Co60 megavoltage radiotherapy machine. The animals were operated on under general anesthesia 2 weeks later. During the operation, Femoral microarterial anastomoses were performed. In Group 2, the rats were irradiated and operated on as in Group I, on the same day. In Group 3, the rats were operated on and, 2 weeks later, irradiated, as in Group 1. In Group 4 (control group), the rats were operated on as in Group 1 No radiotherapy was performed. All rats were explored under general anesthesia after 1 month. Patency was evaluated, and the anastomotic sites were taken for histopathologic and electron microscopy studies.