1. Phencyclidine (PCP) is an inhibitor of the nicotinic acetylcholine receptor (AChR) with characteristics of an open-channel blocker. The location of PCP binding site on the AChR molecule is unknown. 2. PCP inhibits the AChR from electric organ with a higher potency than muscle AChR. To find the molecular basis of this difference, we expressed the two native and six hybrid receptors, and two receptors containing mutated mouse gamma subunits in Xenopus laevis oocytes. The inhibition of ACh-induced current in these receptors by PCP was studied using whole-cell voltage-clamp. All hybrid receptors generated robust ACh-induced currents, while incomplete receptors (gamma-less or delta-less) did not. 3. PCP potency was higher on hybrids containing Torpedo beta and gamma subunits regardless of the alpha and delta subunit origin. A mouse gamma subunit containing the asparagine 6' to the serine mutation in the M2 segment conferred a high sensitivity to PCP. 4. These results support the conclusion that the amino acid residues at the position 6' of the M2 segments contribute to the PCP potency difference between Torpedo and mouse receptors. 5. Another noncompetitive inhibitor of the AChR, the cembranoid eupalmerin acetate (EUAC), also inhibited the electric organ receptor with a somewhat higher potency than muscle AChR. However, the IC50 values for EUAC inhibition of hybrid receptors did not follow the pattern observed for PCP. Therefore, these two inhibitors interact differently with the AChR molecule.