The mineralocorticoid receptor (MR) differs from the other steroid receptors in that it responds to two physiological ligands, aldosterone and cortisol. In epithelial tissues, aldosterone selectivity is determined by the activity of 11β-hydroxysteroid dehydrogenase type II. The aldosterone-induced genes that mediate sodium flux in epithelial tissues are now well characterised. In other tissues, including the heart and regions of the CNS, cortisol is the primary ligand for the MR; in some tissues cortisol may act as an antagonist. Clinical trials have demonstrated the potential of MR antagonists in the treatment of cardiovascular disease however their use has been limited by concurrent hyperkalaemia. The mechanisms of these therapeutic effects are currently being addressed by the use of animal models including tissue-specific deletions of the MR. In order to better target the MR an understanding of the structural determinants of tissue and ligand-specific MR activation is being sought. We have focused on interactions of the ligand-binding domain (LBD) with ligand, with the N-terminal domain and with putative co-regulatory molecules. Both agonist and antagonist binding has been characterised using chimera between the human (h)MR LBD and both the glucocorticoid receptor and the zebra fish (z)MR together with molecular modelling. An interaction between the N-terminus and C-terminus/LBD (N/C-interaction) observed in the MR is aldosterone-dependent but is unexpectedly antagonised by cortisol and DOC in the hMR but not the zMR. Nuclear receptor mediated transactivation is critically dependent on, and modulated by, co-regulatory molecules. Yeast-2-hybrid screens with the MR LBD have identified proteins which interact in the presence of either aldosterone or cortisol but not both. These have been confirmed as coactivators of the full-length hMR in a transactivation assay. The successful identification of ligand-specific interactions of the MR may provide the basis for the development of novel MR ligands with tissue specificity.