The intrinsic tryptophan fluorescence signal of the full-length nuclear receptor hVDR was used to directly determine the dissociation constants, K-d, Of two ligands yielding K-d, = 32 nM for 1 alpha ,25(OH)(2)D-3 and K-d = 322 nM for 25(OH)D-3. Ligand binding was accompanied by a conformational change in the a-helical part of hVDR as revealed by CD spectroscopy. In addition, the presence of calcitriol was found to be a necessary prerequisite for the homodimerisation of hVDR which was monitored using fluorescence anisotropy. We conclude that the observed ligand-induced structural change of hVDR is conditional for dimerisation of the protein.