The diversity in nature has long been and still is one of the biggest resources of pharmaceutical lead compounds and many natural products often exhibit biological activity against unrelated biological targets, thus providing us with starting points for pharmacological analysis. Natural peptides of great number and diversity occur in all organisms from plants to microbes to man. Examples for such rich and yet largely untapped libraries of bioactive compounds are animal venom peptides, invertebrate peptide hormones or plant defense peptides. Our goals are to discover and characterize novel oxytocin- and vasopressin neuropeptide analogs in invertebrates, study their function, determine their pharmacological activity, and use them as probes to design peptides to develop ligands for human G protein-coupled receptors. Using an interdisciplinary approach by combining physiology, pharmacology and peptide chemistry, we are aiming to generate selective, potent and stable peptide ligands and may be useful for the treatment of a wide range of challenging, but yet untreated diseases.