The transient expression of many different genes is mediated by the inducible transcription factor p50-p65 NF kappa B, which in turn is regulated by complex formation with its inhibitor I kappa B alpha. We describe here that in porcine aortic endothelial cells, either IL-1 alpha, TNF alpha or LPS upregulates an inhibitor of NF kappa B which we refer to as ECI-6. ECI-6 is by structural and functional criteria an I kappa B alpha protein, the porcine homologue of MAD-3, pp40 and RL/IF-1. We have studied the promoter of the ECI-6/I kappa B alpha gene and provide three lines of evidence that its expression is directly regulated by NF kappa B. First, the 5' regulatory region of ECI-6/I kappa B alpha contains two sites that bind NF kappa B in electrophoretic mobility shift assays. Second, expression following transfection of an ECI-6/I kappa B alpha promoter-luciferase reporter construct is dependent on a co-transfected NF kappa B-p65 subunit. Third, pretreatment of endothelial cells with antioxidants, agents that inhibit activation of NF kappa B, inhibit the expression of ECI-6/I kappa B alpha. We conclude that the regulated expression of ECI-6/I kappa B alpha could represent a novel feedback mechanism by which NF kappa B downregulates its own activity after transient activation of target genes has been achieved.