To determine the endurable radiation dose before bronchial anastomosis, the bronchial circulation of mongrel dogs was assessed after bronchoplastic surgery conducted after various single doses of radiation. Plastic surgery of the right main bronchus was carried out 1 week after cobalt 60 irradiation to the right hilar region. Bronchial blood flow was measured with laser Doppler velocimetry 7, 14, and 30 days after operation. Animals were killed 30 days after operation and the region of the bronchial anastomosis was examined histologically. The animals were divided into five groups: group A (control group without irradiation), group B (8 Gy irradiation), group C (16 Gy), group D (19 Gy), and group E (24 Gy). Wound healing of the anastomosed bronchus was excellent in groups A and B. In group A, bronchial blood flow did not drop below 80% of the preoperative level throughout the postoperative observation period. In group B, bronchial blood flow on day 7 after operation was only 65% of the preoperative level, but blood flow had returned to the preoperative level by day 30. In groups C, D, and E, bronchial blood flow was less than 60% of the preoperative level on day 30. There was a high prevalence of radiation-induced morbidity in the region of anastomosis in groups C, D, and E. Postoperative recovery of bronchial blood flow and wound healing of the anastomosed bronchus were thus delayed in dogs that had received high doses of radiation before operation. We conclude that the safe dose of preoperative radiation in clinically used fractionation is 36 Gy or less (corresponding to a single dose of 16 Gy or less in dogs).