Nicotinic acetylcholine receptor protein (nAChR) has been solubilized from rat cerebral cortices by extracting a crude membrane fraction with the nonionic detergent Triton X-100 (polyoxyethylene-p-t-octylphenol). The solubilized nAChR was partially purified by affinity chromatography (Naja naja siamensis alpha-toxin affinity arm, linked to Sepharose 4B) and characterized by binding of 125I-labeled alpha-bungarotoxin. The reaction of labeled toxin and nAChR appears to be second order with a rate constant (k1) equal to 0.38 X 10(5) M-1 S-1 at 20 degrees. The toxin-nAChR complex dissociates with a dissociation rate constant (k-1) of 1.23 X 10(-5) S-1 at 20 degrees (t 1/2 = 15.6 h). The kinetically determined dissociation constant (Kd) for the complex is 3.24 X 10(-10) M. A variety of cholinergic ligands were studied for their ability to inhibit binding of labeled toxin. The results indicate that the brain receptor is indeed nicotinic. The s20, w and v of the toxin-nAChR complex in 0.1% Triton were determined by velocity sedimentation in D2O and H2O sucrose gradients. The values are 12.9 S and 0.80 cm3 g-1. The Stokes radius of the complex determined by gel filtration equals 7.5 nm. The Mr of the complex calculated from the hydrodynamic parameters, and corrected for bound detergent, equals 357,000.