Properties of mammalian central nervous system binding sites for curaremimetic neurotoxins are investigated with the Simonsen-Albino rat and 125I-labeled alpha-bungarotoxin or the principal neurotoxin from Naja naja siamensis. Evidence is presented that high-affinity toxin binding sites are distributed as expected for a synaptic neurotransmitter receptor, display distinct nicotinic cholinergic pharmacology, and are sensitive to preincubation with nicotinic agonists. Affinity of toxin sites for agonists is altered by specific sulfhydryl/disulfide modification and by Ca2+, and sites may be labeled with the nicotinic acetylcholine receptor affinity reagent bromoacetylcholine. New data are also presented indicating that toxin binding sites with K' values of approximately 1 nM and approximately 100 nM may be detected on rat brain crude mitochondrial fractions. Evidence is also reported suggesting the existence of two classes of toxin binding site disulfides/sulfhydryls, which interact with affinity reagents and nonspecific alkylating agents and are located proximal and distal, respectively, to the acetylcholine binding site. The results indicate that central nervous system (CNS) toxin binding sites share significant biochemical homology with nicotinic receptors from the periphery and provide a foundation for further study of toxin binding site biochemistry and the relationship between toxin sites and authentic CNS nicotinic acetylcholine receptors.