On the basis of endoscopic investigations, the reaction to X-radiation of the hindgut of the rat could be divided into four phases. Their courses after different radiation doses as well as after dietary influence are described. This enabled us - besides the acute (diarrhoea) and chronic (diarrhoea followed by intestinal occlusion) phases shown by clinical examination - to demonstrate the early vascular reaction (first week after irradiation) as well as the asymptomatic interval between the acute and chronic phases. The interval phase was found to be of special importance for prognosis and therapy, because all animals who died later on of intestinal occlusion developed already during this phase circular ulcerations which could be demonstrated by endoscopy.