Dynorphin A (Dyn A) is an endogenous ligand for the opioid receptors with preference for the κ opioid receptor (KOR), and its structure-activity relationship (SAR) has been extensively studied at the KOR to develop selective potent agonists and antagonists. Numerous SAR studies have revealed that the Arg residue is essential for KOR activity. In contrast, our systematic SAR studies on [des-Arg]Dyn A analogues found that Arg is not a key residue and even deletion of the residue does not affect biological activities at the KOR. In addition, it was also found that [des-Arg]Dyn A(1-9)-NH is a minimum pharmacophore and its modification at the N-terminus leads to selective KOR antagonists. A lead ligand, 14, with high affinity and antagonist activity showed improved metabolic stability and could block antinociceptive effects of a KOR selective agonist, FE200665, in vivo, indicating high potential to treat KOR mediated disorders such as stress-induced relapse.