Isolation, synthesis and therapeutic use of compounds and related compositions based on a new class of conopeptides comprising the modified amino acid .gamma.-hydroxyvaline (Hyv=V*). These isolated peptides are the first known example of a naturally occurring polypeptide chain containing Hyv. The active peptides, termed .gamma.-Hydroxyconophans are heavily hydroxylated small peptides. These peptides contain a definitive structural motif which is a double modification of the polypeptide chain (.gamma.-OH-Hyv-D-Trp).