Morphine, an opium alkaloid derived from the poppy, Papaver somniferum, has been one of the most commonly used analgesics in both clinical and experimental fields despite its addictive properties. In 1975, after a specific opiate receptor had been demonstrated, two endogenous opioid peptides, methionine enkephalin (Met-enk) and leucine enkephalin (Leu-enk), which bind specifically to the same receptor as do opiates such as morphine, were found in the mammalian brain. Since then, many new opioid peptides have been found not only in the central nervous system but also in peripheral organs such as the adrenal gland and intestines. In physiological and pharmacological studies of opioid peptides it has generally been found that these endogenous peptides have analgesic properties.