Torpedo californica postsynaptic membrane fragments were treated with base, which resulted in membranes that were depleted of many nonacetylcholine receptor polypeptides and contained acetylcholine receptor subunits of Mr 40,000, 50,000, 60,000, and 65,000 (Raftery, M.A., Vandlen, R.L., Reed, K.L. & Lee T. (1975) Cold Spring Harbor Symp. Quant. Biol. 40, 193-202). A 43,000-Mr polypeptide and some other components were quantitatively extracted. Base-treated membranes retained the capacity to bind [3H]perhydrohistrionicotoxin and the local anesthetics dibucaine and tetracaine. The regulation of this binding by carbamylcholine, as well as the kinetic mechanism of perhydrohistrionicotoxin binding, was unchanged. [3H]Perhydrohistrionicotoxin binding activity was largely reconstituted from 2% sodium cholate extracts of base-treated membranes. Therefore, the perhydrostrionicotoxin binding site appears to be located on one or more of the acetylcholine receptor polypeptides, and the reconstitution of that binding site from detergent extracts does not require the presence of a 43,000-Mr polypeptide.