NF-kappa B is a key transcription factor of lymphocytes and macrophages with important regulatory functions in the immune system and inflammatory processes. These functions are at least partially based on its ability to regulate the promoters of a variety of genes whose products, such as cytokines, adhesion molecules and acute phase proteins, are critical for inflammatory processes. In the present study, we describe a method to treat established intestinal inflammation by local or systemic application of antisense phosphorothioate oligonucleotides targeting the translation start site of the p65 subunit of NF-kappa B. Specific downregulation of p65 by administration of antisense phosphorothioate oligonucleotides to mice with experimental colitis abrogated clinical and histological signs of mucosal inflammation. In contrast, control nonsense oligonucleotides were virtually inactive. Whereas systemic application of high doses (> 1500 micrograms) of p65 antisense oligonucleotides had striking toxic side effects, local application at all dosages tested did not result in toxic effects. The data provide direct evidence for the central importance of NF-kappa B p65 in experimental models of chronic intestinal inflammation and suggest the potential therapeutic utility of local p65 antisense treatment for patients with chronic intestinal inflammation.