It is demonstrated how a definite part of the rectum of the female Wistar rat can be selectively irradiated without surgery. The effect of X irradiation on the rectum is in the acute phase (second week after irradiation) a haemorrhagic radiation proctitis and in the chronic phase a rectal obstruction. The dose-effect relationship using rectal obstruction as the end point is a sigmoid curve with a ED10 of 17.50 Gy, ED50 of 21.5 Gy and ED90 of 27.5 Gy. Latency time decreases exponentially with increasing dose and is 150 days at the ED50. Histological findings show mucosal glands and cysts within the submucosa and a severe submucosal radiation within the submucosa and a severe submucosal radiation fibrosis which results in the rectum becoming a rigid and non-elastic tube which leads to a functional block in transport of faeces.