Notes: Abstract: Dopamine receptor binding proteins were sol-ubilized with the detergent 3–(3–cholamidopropyl) dimethylammonio - 2 - hydroxy - 1– propanesulfonate (CHAPSO) from bovine and rat striatal membranes. The binding of the dopamine antagonist [3H]spiroperidol ([3H]Spi) to the solubilized dopamine receptors was determined by the polyethyleneglycol method. The CHAPSO-solubilized dopamine receptor binding proteins remain in the supernatant fraction following centrifuga-tion at 100,000 ×g for 2 h. The CHAPSO-solubilized dopamine receptor proteins, as well as the prelabeled [3H]Spi-receptor protein complex, bind specifically to wheat germ agglutinin (WGA)-agarose columns, which is consistent with an identification as glycoproteins. HPLC analysis of the CHAPSO-solubilized, prelabeled [3H]Spi-receptor protein complex (CHAPSO preparation) reveals association with a high molecular weight form, indicating the formation of aggregates and/or micelles. Treatment of the WGA-agarose-bound [3H]Spi-receptor protein complex with digitonin (CHAPSO-digitonin preparation) results in dissociation of the high molecular weight form into lower molecular weight forms. The HPLC profile of the prelabeled [3H]Spi-receptor complex in the CHAPSO-digitonin preparation reveals two radioactive peaks. The major peak had a retention time of 16 min, corresponding to an apparent MW of 175,000, whereas the minor peak had a retention time of 21 min, corresponding to an apparent MW of 49,000. The CHAPSO-solubilized dopamine receptor binding proteins are sensitive to modulation by GTP, indicating that the association with the GTP binding component is preserved in the “soluble” state. The potencies of dopamine antagonists and agonists for inhibiting the binding of [3H]Spi to CHAPSO-solubilized dopamine receptor proteins are similar to those for membrane-bound proteins. Chronic treatment with haloperidol increases the Bmax, and does not change the KD for [3H]Spi in the CHAPSO-solubilized and in the membrane-bound preparations. Thus, the CHAPSO-solubilized dopamine receptor proteins retain the binding characteristics of the supersensitive membrane-bound dopamine receptors.