In an experimental study, resembling a clinical trial of preoperative irradiation, 10 + 10 Gy was given to the pelvic and lower abdominal region of rats with a 4-day interval. The early effect on the colonic wall was evaluated by myeloperoxidase activity and hydroxyproline content of the bowel wall and correlated to histological findings. Groups of animals were followed up to eight months after irradiation for evaluation of later effects. General effects of irradiation were seen as low WBC during the first week and delayed body weight development up to two months after irradiation. Local effect in the colonic wall was noted as an increase in myeloperoxidase activity (indicating a leucocyte accumulation) in irradiated parts of colon during the first 11 days and again significantly elevated after two months in parts of colon, irradiated as well as protected. This correlated well with histological findings of inflammatory reaction, atypia and dysplasia during the first 10 days after irradiation but not at two months after irradiation. Hydroxyproline content was not affected. There were no major complications due to irradiation seen in the late course of the study period.