Regulation of NF-κB transactivation function is controlled at several levels, including interactions with coactivator proteins. Here we show that the transactivation function of NF-κB is also regulated through interaction of the p65 (RelA) subunit with histone deacetylase (HDAC) corepressor proteins. Our results show that inhibition of HDAC activity with trichostatin A (TSA) results in an increase in both basal and induced expression of an integrated NF-κB-dependent reporter gene. Chromatin immunoprecipitation (ChIP) assays show that TSA treatment causes hyperacetylation of the wild-type integrated NF-κB-dependent reporter but not of a mutant version in which the NF-κB binding sites were mutated. Expression of HDAC1 and HDAC2 repressed tumor necrosis factor (TNF)-induced NF-κB-dependent gene expression. Consistent with this, we show that HDAC1 and HDAC2 target NF-κB through a direct association of HDAC1 with the Rel homology domain of p65. HDAC2 does not interact with NF-κB directly but can regulate NF-κB activity through its association with HDAC1. Finally, we show that inhibition of HDAC activity with TSA causes an increase in both basal and TNF-induced expression of the NF-κB-regulated interleukin-8 (IL-8) gene. Similar to the wild-type integrated NF-κB-dependent reporter, ChIP assays showed that TSA treatment resulted in hyperacetylation of the IL-8 promoter. These data indicate that the transactivation function of NF-κB is regulated in part through its association with HDAC corepressor proteins. Moreover, it suggests that the association of NF-κB with the HDAC1 and HDAC2 corepressor proteins functions to repress expression of NF-κB-regulated genes as well as to control the induced level of expression of these genes.