Two penta-peptides with morphine-like properties have recently been found in the nervous system and gastrointestinal tract. These peptides known as enkephalins are unevenly distributed in the brain, the highest concentration being in the globus pallidus. The enkephalins are found to have analgesic properties and are also anti-tussive, anti-diarrheal and able to stimulate release of prolactin and growth hormone. Synthetic analogues of the enkephalins containing specific D-amino acid substitutions have been developed and show a wide spectrum of activities.