The synthesis and characterization of the conopeptide, SO-3, originally derived from Conus striatus is reported. It contains 25 amino acid residues and three disulfide bridges and manifests 72% sequence identity with MVIIA, an N-type Ca2+ channel inhibitor of high analgesic activity. We evaluated SO-3 in several mouse models of pain. The results indicate that SO-3 is a potent, nonaddictive, analgesic agent.