Female Wistar rats were treated with single exposure irradiation to 2 cm of distal colon to cause radiation proctitis. All animals were evaluated by examination, colonoscopy and histologic evaluation for changes post-irradiation. Exposures of 10, 12.5, 15, 17.5, 20, 22.5, 25, 27.5 and 30 Gy caused dose-related clinical and histologic changes peaking at 7 to 15 days post-exposure. Rats treated with 20 Gy were colonoscoped and biopsied daily and showed sequential post-irradiation endoscopic changes ranging from mucosal edema and mild inflammatory changes to erosion and ulcers. Histologically, crypt abscess and mural wall necrosis similar to changes found in the human rectum after radiotherapy were noted. Treatment with nonsteroidal anti-inflammatory agents, (aspirin, indomethacin, piroxicam), misoprostol (a prostaglandin E1 analogue), or sucralfate (an anti-ulcer agent) did not ameliorate nor exacerbate radiation proctitis in rats exposed to 22.5 Gy. We conclude from these data that the female Wistar rat is a good model for studying radiation proctitis because endoscopic, histologic, and clinical changes seen post-exposure closely resemble those found in man.