||Cone snails ( Conus ) are carnivorous marine gastropods that feed on such diverse prey as marine worms, molluscs, and most remarkably, fish. They have accomplished this by developing a vast number of specialized toxins that selectively target particular receptors and ion channels in the nervous systems of their prey. These Conus toxins, collectively called conotoxins, are usually small, highly disulfide-rich peptides. The primary aim of the work presented in this dissertation was to characterize the activities of delta- andmu-conotoxins. Peptides belonging to these conotoxin families selectively affect tetrodotoxin-sensitive (TTX-sensitive) and tetrodotoxin-resistant (TTX-resistant) voltage-gated sodium channels (VGSCs). An understanding of the pharmacology and physiological effects of these conotoxins should allow them to be used as functional probes for VGSCs. Chapter 3 describes the activities of delta-conotoxins PVIA and SVIE from the fish-hunting cone snails Conus purpurascens and Conus striatus respectively. Both of these toxins inhibit the inactivation kinetics and affect the voltage-sensing properties of sodium channels in frog sympathetic neurons. While the effects of delta-PVIA were reversed by washing, those of delta-SVIE were largely irreversible over the time course of these experiments. We conclude that the effects of delta-PVIA and delta-SVIE are remarkably similar to those of site-3 VGSC toxins from other venomous organisms. Chapters 4 and 5 describe the inhibition of TTX-sensitive and TTX-resistant VGSCs in frog sympathetic and dorsal root ganglia neurons by mu-conotoxins. Chapter 4 describes the activity of ?-conotoxin SmIIIA, which may be the first selective antagonist of TTX-resistant VGSCs. Chapter 5 compares SmIIIA's activity with other "new" and "classic" mu-conotoxins. It is concluded that the classic ?-conotoxin, PIIIA, does not inhibit TTX-resistant VGSCs, while "new" mu-conotoxins SmIIIA, S3.2, and Kn3.1 do. Furthermore, S3.2 and Kn3.1 may be even more selective than SmIIIA for TTX-resistant VGSCs. Chimeras between SmIIIA and PIIIA were generated. These chimeras reveal that the C-terminal half of "new" mu-conotoxins contains sequence features important for their selective inhibition of TTX-resistant VGSCs.