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, norepinephrine) 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.