In the past year, opiate receptors have been purified to apparant homogeneity from the membranes of neuroblastoma x glioma hybrid cells. The receptors were first reacted with the newly developed affinity ligand, super-FIT, labelled with tritium to high specific activity. Purification was accomplished by lectin and antibody affinity chromatography techniques and preparative polyacrylamide gel electrophoresis. The purified material migrates with an apparant molecular weight of 55,00 - 58,000 depending upon the electrophoretic system. We have also purified the GTP binding protein, Ni, to homogeneity from bovine brain membranes. The material has been characterized by analytical ultracentrifugation, amino acid analysis, subunit composition studies and by its activity as a substrate for pertussis toxin and in reconstitution of opiate inhibition of adenylate cyclase in pertussis toxin treated cells. We have also determined that the long term increase in adenylate cyclase activity resulting from chronic exposure to opiates is a reflection of a decrease in the cyclase associated GTPase activity, perhaps as the result of a modification of Ni. Receptor down-regulation, induced by highly efficacious agonists only, does not appear to be related to changed Ni functions.