Mesalamine (5-ASA) is effective in the treatment of inflammatory bowel diseases. However, the mechanisms of action of 5-ASA remain unclear. IEC-6 and IRD-98, nontransformed rat small intestinal epithelial cell lines, were used to examine the effect of 5-ASA on the expression of manganese superoxide dismutase (MnSOD). Rats were given 5-ASA enemas to determine the effect on colonic MnSOD expression. Treatment with 5-ASA at 0.02 or 2 mg/ml induced MnSOD mRNA levels 2.67-fold or 5.66-fold, respectively. Inhibition of 5-lipoxygenase activating protein with MK-886 or cyclooxygenase with indomethacin did not influence the level of MnSOD mRNA. Nuclear run-on experiments demonstrated an increase in de novo transcription following treatment with 5-ASA. MnSOD protein levels were induced 2-fold at 24 h and 4.23-fold at 48 h following treatment with 1 mg/ml 5-ASA. 5-ASA increased MnSOD 1.7-fold in vivo. Pretreatment with 5-ASA significantly protected IRD-98 cells from tumor necrosis factor- cytotoxicity. This is the first example of transcriptional gene regulation by 5-ASA. The induction of MnSOD by 5-ASA may contribute to the therapeutic mechanism of 5-ASA.