A number of pathogenic and proinflammatory stimuli, and the transforming growth factor-β (TGF-β) exert opposing activities in cellular and immune responses. Here we show that the RelA subunit of nuclear factor κB (NF-κB/RelA) is necessary for the inhibition of TGF-β-induced phosphorylation, nuclear translocation, and DNA binding of SMAD signaling complexes by tumor necrosis factor-α (TNF-α). The antagonism is mediated through up-regulation of Smad7 synthesis and induction of stable associations between ligand-activated TGF-β receptors and inhibitory Smad7. Down-regulation of endogenous Smad7 by expression of antisense mRNA releases TGF-β/SMAD-induced transcriptional responses from suppression by cytokine-activated NF-κB/RelA. Following stimulation with bacterial lipopolysaccharide (LPS), or the proinflammatory cytokines TNF-α and interleukin-1β (IL-1β, NF-κB/RelA induces Smad7 synthesis through activation of Smad7 gene transcription. These results suggest a mechanism of suppression of TGF-β/SMAD signaling by opposing stimuli mediated through the activation of inhibitory Smad7 by NF-κB/RelA.