Most small peptide hormones and neurotransmitters are highly flexible, conformationally labile molecules in aqueous and other environments. Thus efforts to determine the relationships between conformational properties of these peptides in aqueous and other solvents and their biological activities at membrane receptors have been difficult and of limited success. One approach which may provide a more rational basis for conformation-activity relationships is the design of conformationally restricted, semi-rigid analogs of the native peptides which still possess high potency and/or antagonist properties. In addition to the increased likelihood that the conformational properties determined for these derivatives in aqueous or other solvent environments will have biological relevance, such analogs are likely to have higher specificity for particular receptors, greater in vivo stability, and perhaps even oral activity. The application of this approach to the design of highly potent oxytocin antagonists is discussed with particular emphasis on the conformational and dynamic properties which appear to differentiate agonist and antagonist analogs. The results of these studies are briefly compared with similar studies with somatostatin, angiotensin, bradykinin, alpha-melanotropin and enkephalin, and discussed in terms of likely further developments.