The two disulfide bonds of alpha-conotoxin ImI, a peptide antagonist of the alpha 7 nicotinic acetylcholine receptor (nAChR), were systematically replaced with isosteric redox-stable cystathionine thioethers. Regioselective thioether formation was accomplished on solid support through substitution of a gamma-chlorohomoalanine by an intramolecular cysteine thiol to produce hybrid thioether/disulfide analogues (2 and 3) as well as a dual cystathionine analogue (4) that were found to be structurally homologous to alpha-conotoxin ImI by (1)H NMR. The antagonistic activity at the alpha 7 nAChR of cystathionine analogue 3 (pIC(50) = 6.41 +/- 0.09) was identical to that of alpha-conotoxin ImI (1, pIC(50) = 6.41 +/- 0.09), whereas those of 2 (pIC(50) = 5.96 +/- 0.09) and 4 (pIC(50) = 5.89 +/- 0.09) showed a modest decrease. The effect of oxidation of the thioethers to sulfoxides was also investigated, with significant changes in the biological activities observed ranging from a >30-fold reduction (28=O) to a 3-fold increase (38=O(B)) in potencies.