Chronic or repeated administration of kappa opioid agonists leads to tolerance to the subsequent drug administration. The mechanisms underlying tolerance are complex and changes at the receptor level contribute in part to the development of tolerance. This review focuses on agonist-induced phosphorylation, desensitization, internalization and down-regulation of the kappa opioid receptor. In vivo studies on the rat and guinea pig brains are reviewed, followed by in vitro investigations on cells and tissues endogenously expressing the kappa opioid receptor. The bulk of the article describes the studies performed after cloning of the opioid receptors on regulation and trafficking of the kappa opioid receptors (KORs) expressed in various cell systems. Topics reviewed and discussed include biochemical mechanisms of desensitization, internalization and down-regulation, differences in the regulation of the rat and the human kappa opioid receptors (rKOR and hKOR, respectively) and the structural basis for the species variations, differential abilities of agonists in inducing regulation of the hKOR, the relationship (or the lack thereof) of KOR internalization to activation of p42/p44 mitogen-activated kinase and to adenylyl cyclase superactivation, the role of the PDZ domain-containing protein NHERF-1/EBP50 in the trafficking of the hKOR and the relationship between receptor phosphorylation and tolerance development in mice. There are still questions remained to be answered. Among the issues to be resolved are the signals that direct the sorting of internalized hKORs to the recycling and degradation pathways, the recycling pathway(s) of the internalized hKOR, the molecular bases of differential regulation of the KORs by agonists and the occurrence of agonist-induced KOR internalization occur in vivo and, if so, its role in tolerance and dependence.