The results suggest that: 1) Zinc ions may chelate the histidines critical for the agonist binding preventing hydrogen bonds between nonprotonated nitrogen atom of His-251 and the exocyclic N6-H in CHA or CCPA molecule and between His-278 and -OH of the ribose ring. This mechanism can explain the reduction in the number of binding sites without changing the affinity. 2) Cadmium ions may oxidize cysteine SH-groups. The redox reaction between Cd2+ and receptor thiols may result in binding of the metal into stable (di)thiol-cadmium complexes rather than in the formation of disulfide and liberation of the reduced metal. This mechanism can justify the conformational modifications of the receptor molecule producing the decrease in affinity.