Within the spectrum of the characteristic pharmacological activities (oxytocic (O), milk-ejecting (ME), antidiuretic (A), pressor (P) associated with the known natural and synthetic analogs of oxytocin and vasopressin it is possible to discern patterns of selectivity of these types: 1) interpeptide-like (a) O/A, (b) O/P; 2) intraoxytocin-like (a) O/ME; (b) ME/O; 3) intravasopressinlike (a) A/P, (b) P/A. Consideration of structural modifications of oxytocin or vasopressin, which individually or in combination give rise to peptides possessing enhanced selectivity of a given type, can in some cases provide a rational basis for the design of peptides with even greater selectivity. [1-Deamino-4-valine-8-D-arginine]vasopressin, the most highly specific antidiuretic peptide known to date, was designed in this fashion. By contrast, intraoxytocin-like selectivity, is manifested to only a minor degree in all peptides studied to date. Enhanced interpeptide-like selectivity of the type 1a (O/A; O/P) is readily attainable by specific single substitutions at positions 4 or 7 in oxtocin. Substitution of threonine in the 4 position of the oxytocic antagonist [1-deaminopenicillamine]oxytocin brought about a threefold enhancement in oxytocic inhibitory activity. Thus [1-deaminopenicillamine-4-threonine]oxytocin (dPTOT) is the most potent antagonist of the in vitro oxytocic response to oxytocin known to date. Thus analysis of the pharmacological data from over 300 analogs of oxytocin and vasopressin allows the delineation of those structural modifications that can optimize selectivities. The potential and limitations of this approach for the design of peptides possessing desired agonistic or antagonistic selectivity for potential clinical use and for studies on oxytocin and vasopressin receptors is discussed.