A colitis-like state induced in Guinea Pigs fed degraded carrageenan orally. By means of a combined semimacroscopic and histologic technique the course of the disease was followed during 28 days. The changes were primarily seen and became most prominent in the caecum. The first lesions were observed following 24 hours of treatment as small rounded foci initially with degenerative changes and inflammation in the surface epithelium, later forming superficial focal ulcerations. Ulcerative changes gradually progressed during the experiment, forming linear and later large, geographical ulcerations. Topographically the ulcerative process was strongly related to the larger submucosal vessels. Nonulcerated parts of the mucosa were not changed until following 7-14 days of treatment. The mucosa became bulging, granulated and finally villus-like. Accumulation of macrophages was found under the surface epithelium after 7-17 days. Possible pathogenetic mechanisms are discussed, especially the development of the early lesions and the significance of the macrophages.