Combinatorial regulation of transcription involves binding of transcription factors to DNA as well as protein-protein interactions between them. In this paper, we demonstrate the existence of a mutual transcriptional antagonism between the thyroid hormone receptor (TR) and the cyclic AMP response element binding protein (CREB), which involves a direct association of both transcription factors. TR inhibits transcriptional activity of CREB and represses activation of cAMP response element (CRE)-containing promoters. TR does not bind to the CRE in vitro, but in vivo the liganded receptor is tethered to the promoter through protein-protein interactions. In turn, expression of CREB reduces TR-dependent transcriptional responses. The association of TR with CREB inhibits the ability of protein kinase A to phosphorylate CREB at Ser133, and leads to a reduction in the ligand-dependent recruitment of the p160 coactivators by TR. These results indicate the existence of a transcriptional cross-talk between CREB and TR signalling pathways, which can have important functional consequences.