The agonist binding site of nicotinic acetylcholine receptors (AChRs) includes a disulfide bond that is easily reduced with dithiothreitol to a pair of thiols, and can be then either reoxidized with dithiobis(nitrobenzoic acid) (DTNB) or irreversibly alkylated with bromoacetylcholine (BAC). Aromatic trivalent arsenicals form stable complexes with pairs of appropriately-spaced thiols, but not single thiols. Furthermore, once complexed in proteins, trivalent arsenicals can be removed with dimercaptans, such as 2,3-dimercaptopropanesulfonic acid (DMPS). In an effort to develop reagents that will covalently, yet reversibly label AChRs, we investigated the effects of two model arsenicals, p-aminophenyldichloroarsine (APA) and 4-bromoacetyl-aminophenylarsenoxide (BAPA) on two types of nicotinic receptors: AChRs from Torpedo electroplax and neuronal receptors from chick retina. APA and BAPA significantly decrease the number of 125I-alpha-bungarotoxin binding sites in reduced Torpedo AChRs. Furthermore, arsenylation of neuronal and Torpedo receptors with APA or BAPA (1) prevents reoxidation with DTNB, (2) is reversible with DMPS, and (3) protects against irreversible alkylation by BAC. In Torpedo receptors, the EC50 of protection against BAC alkylation with APA or BAPA is approximately 30 nM. APA arsenylation of Torpedo receptors persists up to 20 h, but can be reversed at any time with DMPS. These results suggest that heterobifunctional arsenicals could anchor labeling groups in the agonist binding site in order to map the agonist binding site, quantitate receptors, or purify and reconstitute functional receptors.