The vitamin D receptor mediates the signal of 1 alpha, 25-dihydroxyvitamin D3 by binding to vitamin D responsive elements in DNA as a homodimer or as a heterodimer composed of one vitamin D receptor subunit and one retinoid X receptor subunit. We have mapped the dimerization interfaces of the vitamin D receptor that is involved in homo- or heterodimer formation in the absence of DNA. While deletion of the first zinc finger region of vitamin D receptor diminished homodimerization activity, it did not affect heterodimerization. In contrast, a deletion just beyond the zinc finger region affected heterodimerization with retinoid X receptor, but not homodimerization. The zinc finger region alone could form a homodimer with full-length vitamin D receptor, but not a heterodimer with retinoid X receptor. The carboxy-terminal region was also necessary for heterodimer formation. This region showed only a weak dimerization activity in the absence of ligand, but this was dramatically increased in the presence of ligand for both homo- and heterodimerization. These results suggest that the vitamin D receptor has at least three dimerization interfaces whose functions are apparently distinguishable. These are located in the first zinc finger region, the region just beyond this zinc finger and in the carboxy-terminal region.