Our current understanding of the biological basis of anxiety is limited. Research on affective states including anxiety has focused on the roles of the brain gama-aminobutyric acid (GABA) and monoamine (e.g., serotonin, dopamine, norepi-nephrine) systems. The brain opioid systems have been implicated in the regulation of emotion. In this review, we will focus on the role of dynorphins/kappa opioid receptor (DYN/KOPR) system in anxiety regulation. Recent preclinical research suggests that KOPR antagonists have anxiolytic effects. Together with the roles of this brain system in depression and drug addiction, KOPR is a potential target for new medications.