The steroid hormone aldosterone takes part in the regulation of salt and water homeostasis via the mineralocorticoid receptor (MR). The physiological action of the MR is dependent on its homodimerisation or heterodimerisation with the glucocorticoid receptor. To investigate MR interactions we are developing a bioluminescence resonance energy transfer (BRET) assay. This method analyzes protein interactions measuring the energy transfer between a luminescent donor and a fluorescent acceptor molecule, which are fused to the proteins of interest. In order to validate BRET we examined MR homodimerisation using a luciferase-MR fusion protein as a donor and EYFP-MR as an acceptor and measuring the BRET ratio in the presence and absence of aldosterone. The BRET data were positive for Luc- MR & EYFP-MR, but not for Luc and EYFP-MR. In addition, we measured transactivation activity with a reporter assay. These data also suggest dimerisation of Luc-MR and EYFP-MR. Our aim is to further develop the BRET assay for the MR in order to characterize known protein interactions and possibly detect new binding partners.