Opioid receptors are the members of seven transmembrane G-protein coupled receptors. At least 3 types of opioid receptors, micro, kappa, and delta, have been identified. Human delta-opioid receptor, know as OPRD, DOR, or hDOR, is primarily expressed in brain, especially in the areas of cortex, including olfactory bulb, hippocampus, and amygdale, as well as in basal ganglia and hypothalamus. Human OPRD gene has been mapped at chromosome 1p34.3-p36.1. The endogenous ligands for the OPRD are enkephalins. The third extracellular loop of the OPRD is critical for ligand binding. The ligands binding and signal transduction functions of delta-receptor can be synergized by forming heterodimers with kappa-receptor. Activation of opioid receptor inhibits cAMP production, increases phosphodiesterase activity, and results in analgesia and convulsions. Recent studies have indicated that OPRD is involved in the mechanism of heroin dependence.