In summary, the structure-activity profile for cyclic hexapeptide analogues related to the natural product-derived lead 1 shows that high levels of oxytocin receptor affinity can be realized with certain amino acids at the 2and 4-positions and that aqueous solubility can be increased substantially by introducing basic groups at the 5- and 6-positions. Several potent and selective oxytocin receptor ligands which have sufficient aqueous solubility for iv administration have been identified. All of the new high-potency analogues cited here have been characterized as functional oxytocin antagonists similar to L-365,209 (1) in the blockade of oxytocin-stimulated rat uterine contractions in vitro and in vivo.5 Furthermore, these compounds behave as pure antagonists and have shown no oxytocin agonist activity in stimulating phosphatidylinositol turnover in vitro or rat uterine contractions in vitro or in vivo. These detailed studies will be reported separately. Such compounds may have utility as research tools and in certain therapeutic applications.