Reactive oxygen species are thought to play an important role in some bowel diseases. In order to evaluate the participation of nitric oxide and superoxide in such diseases, we examined the expression of nitric oxide synthase (NOS) and superoxide dismutase (SOD) as well as their activities in whole excised colons of rats with colitis induced by intralumenal administration of 2,4,6-trinitrobenzenesulfonic acid. A marked increase in the inducible form of NOS mRNA was detected and NOS activity was coincidentally augmented in the group administered unbuffered TNBS (pH 1.0), in which severe inflammation was revealed by microscopic examination and myeloperoxidase activity of invading neutrophils in the tissues. The levels of the Mn- and Cu,Zn-SOD proteins as well as SOD activity were suppressed, although expression of the Mn-SOD mRNA was enhanced in colitis tissues. The elevation of NOS activity and the suppression of SOD activity occurred concomitantly at the stage of severe inflammation. This would increase peroxynitrite formation from superoxide and nitric oxide and enhance the tissue damage in experimental colitis.