The present study assessed the influence of shortening the overall treatment time (accelerated fractionation) on radiation injury of the small intestine. A rat model which allowed repeated irradiation of a localized segment of small intestine was developed. Young adult male Sprague-Dawley rats were orchiectomized, and a loop of the distal ileum was transposed to the left part of the scrotum. The intestinal segment was irradiated with a total dose of 56 Gy, given in 20 fractions, the total treatment time being either 26, 12, or 7 days (i.e. 1, 2, or 3 fractions per day). Radiation injury was assessed by histopathologic examination at 6 hr and at 2 weeks after the last irradiation. The surgical procedure was without complications. Shortening the overall treatment time by giving more than one radiation dose per day resulted in markedly increased injury both at 6 hr and at 2 weeks. It is concluded that accelerated fractionation results in increased radiation injury of the intestine when compared with standard fractionation. Because there may be a relationship between early and late effects in the intestine, our results also indicate that increased late radiation enteropathy may result from accelerated fractionation.