There is a growing interest in neoadjuvant chemo- and radiotherapy as a treatment modality for colorectal cancer which could affect mechanical and biochemical parameters of anastomotic healing. This study investigated the effect of such protocols on colonic anastomotic healing by evaluating the histopathological parameters. One hundred and sixty male Wistar rats were divided into six groups: a control group (I, n=20), a saline group (II, n=30) which received 1 ml NaC1 intraperitoneally, a sham-irradiated group (III, n=20), a 5-fluorouracil (5-FU) group (IV, n=30), which received 5-FU (20 mg/kg) intraperitoneally for 5 consecutive days, an irradiated group (V, n=40) which received fractionated irradiation to the whole pelvis to a totaldose of 22 Gy, 5.5 Gy per fraction on 4 consecutive days, and a concomitant 5-FU + irradiation group (VI, n=20) which received 5-FU as in group IV and irradiated as in group V. All groups underwent left colonic resection with primary anastomosis, and the last fraction of irradiation and the last injection were given 4 and 3 days before the operation, respectively. Within each group one half of the animals were killed on the third postoperative day and the other half on the seventh postoperative. day. After the resection of the anastomotic segments, histopathological examination was evaluated. Apposition of the wound edges of the mucosa and the muscularis were not affected by the therapy. The level of granulocytes was high, inflammatory exudate and necrosis persisted, granulation tissue formation was delayed, and the levels of macrophages and fibroblasts were low. We conclude that colonic anastomotic healing can be affected by the administration of preoperative chemotherapy, irradiation, and chemoirradiation.