Abstract: Incubation of P₂ membrane preparation of rat brain homogenenate followed by solubilization with Triton-x 100, centrifugation at 100,000g for 1 hr and chromatography by Sephadex-G 200 column of the supernatant led to an elution profile in which opiate receptors and muscarinic receptors were found in the same protein peak. The molecular weight of both receptors were estimated to he 450,000 daltons.Neither opiate nor muscarinic receptors could be obtained in the active from when Triton-x 100 was used as the solubilizing agent, but solubilization with CHAPS was successful. With CHAPS, the yield of opiate and muscarinic receptors were about 23% and 12% respectively. Dilution of the solubilized supernatant with Tris buffer (50 mM, pH 7.5) enhenced the binding capacity of opiate receptor about 25 folds, but when the dilution buffer contained CHAPS (1 mM), there was virtually no increase in binding acivity.