Retinoid X Receptor alpha (RXR alpha), a member of the steroid-thyroid hormone receptor super family, is phosphorylated in vitro by protein kinase A (PKA) and this phosphorylation is inhibited in presence of PKA inhibitory peptide. Analysis of various deletion mutants of RXR alpha indicate that the amino-terminal A/B domain is the target for PKA phosphorylation. An RXR alpha mutant in which serine residue 27 is mutated to alanine is no longer phosphorylated by PKA. In vivo transfection experiments in COS cells indicate that cyclicAMP represses retinoic acid-mediated transcriptional activation of RXR alpha and this repression is mediated by serine 27. These results indicate that serine 27 of RXR alpha is an unique target for phosphorylation by PKA in vitro and it has an important role in the crosstalk between RXR alpha and cyclicAMP signalling pathways.