By oral administration of degraded carrageenan a colitis-like disease can be induced in guinea pigs which almost exclusively affects the caecum. To study the effect of degraded carrageenan on the distal colon and rectum, an ileotransversostomy was performed. In the non-operated group of animals ulcerative lesions and changes in the mucosal surface structure were observed only in the caecum and proximal colon after administration of degraded carrageenan. Animals with ileotransversostomy had similar lesions, but the distal colon and rectum were also severely affected, with formation of crypt abscesses in the rectum. This study indicates that the long transit time in the caecum, which enables bacterial decomposition and/or absorption of degraded carrageenan, is the reason that only the proximal colon is affected in non-operated animals. After ileotransversostomy degraded carrageenan reaches the distal colon in an active form that produces ulcerative changes.