Orally administered dextran sodium sulfate (DSS) produces an acute colitis in rodents. The pathogenesis is unknown but may relate to DSS-mediated toxicity of colonic crypt epithelium and/or DSS-induced inflammation. The purpose of this study was to determine when colonic mucosal inflammation, as indicated by histopathology and intercellular adhesion molecule-1 (ICAM-1) expression, occurs relative to crypt epithelial damage. Groups of eight adult male Wistar rats were administered 5.0% DSS solution in the drinking water for 2-6 days. Clinical signs at 3 days consisted of loose stool, progressing to marked rectal hemorrhage by days 5 and 6 that correlated with marked intraluminal colonic hemorrhage at necropsy. Histological lesions of predominantly the distal colon consisted of multifocal areas of mucosal erosion, reduction in goblet cells, dilated crypts, crypt collapse, increased lamina propria neutrophils, and submucosal edema on days 2 and 3, progressing to locally extensive ulceration and marked mixed inflammatory infiltrates by days 4-6. Enhanced expression of ICAM-1, demonstrated by both immunohistochemical and northern blot analysis, was evident in colonic mucosa as early as day 2, with consistent increases through days 3-6. Results demonstrate that enhanced colonic mucosal endothelial cell ICAM-1 expression is an early event in the inflammatory cascade of DSS-induced colitis.