Each of four basic residues of omega-conotoxin GVIA was replaced with alanine to study the role of basic residues for the binding of this toxin to N-type calcium channels. The activities of these analogs were estimated from the inhibitory action on 125I-omega-conotoxin GVIA binding to chick brain synaptic plasma membranes. The replacement of Arg17, Lys24 and Arg25 resulted in no significant change in the activity and all of the analogs gave the same IC50 value (0.15 nM) as that of native omega-conotoxin GVIA. The inhibitory action of [Ala2]omega-conotoxin GVIA (K2A) was 40-times less potent (IC50 = 5.5 nM); however, full inhibition was achieved at a concentration above 0.1 microM. These results indicate that the Arg residue is not essential for the activity of omega-conotoxin GVIA. The nature of association to ion channels may be different between omega-conotoxin GVIA and mu-conotoxin GIIIA.