Administration of dextran sulfate sodium (DSS) solutions to rats induced colitis which resembled mucosal lesions of human ulcerative colitis. Recent reports have shown that some cytokines are related to the pathogenesis of ulcerative colitis. In the present report, we describe the production of two cytokines in colitis mucosa in this DSS model. Using a cytotoxicity assay and a radioimmunoassay, we observed significant increases in levels of tumor necrosis factor-alpha (TNF-alpha) in the colitis mucosa and detected interleukin-1 alpha in the mucosa of 3 of 5 DSS rats and an increase in TNF-alpha had a tendency to be inhibited by treatment with FK506. Immunohistochemical investigation of DSS mucosa showed that the number of activated T cells increased at the earlier phase of inflammation. Luminol-dependent chemiluminescence values and myeloperoxidase activities were increased in the late phase of colitis and were suppressed by the FK506 treatment. These findings may support the role of TNF-alpha and T-cell activation in the pathogenesis of DSS colitis.