Bicyclization represents an effective method for the introduction of conformational constraints into small, biologically important peptides. Several strategies have been developed for the preparation of bicyclic lactam analogues of α-conotoxin SI, a 13-residue peptide neurotoxin found in cone snail venom. Four analogues of the natural regioisomer of α-conotoxin SI were designed and synthesized, each with one of the two paired cysteines of the parent peptide being replaced by a side-chain lactam bridged glutamic acid/lysine pair. Solid-phase lactamization was studied to determine rates of formation of the two possible loops and to document the extent of dimerization and higher oligomerization. Radioligand binding assays were carried out on all synthesized peptides, including the naturally occurring two-disulfide form, in order to determine their affinities for nicotinic acetylcholine receptors (nAChRs). Replacement of the Cys2-Cys7 loop of α-conotoxin SI with a lactam bridge resulted in complete loss of activity, whereas replacement of the Cys3-Cys13 disulfide loop resulted in a ~60-fold reduction in affinity for one orientation and a ~70-fold increase in affinity for the other. The two active lactam analogues retain the selectivity exhibited by the naturally occurring peptide for the ≃/δ subunit of nAChRs, as judged by competition experiments with the curariform antagonist metocurine.