Abstract: The quantum chemical (INDO) calculations have been undertaken for Leu⁵-enkephalin. The electronic structure was investigated, the active site, the way of action and structure-activity relationship were discussed. It was found that the region of N(1) is the main active site for acceptance of electron, the region of O(51) and O(52) is the main active site for providing electron when interacting with the opiate receptor. Leu⁵-enkephalin was compared with morphine and R31833 in electronic and spatial structure of active site. Study of the results showed that the N (1) in Leu⁵-enkephalin corresponds to the N in morphine and the N (9) in R31833, phenyl of Tyr¹ in Leu⁵-enkephalin corresponds to the phenyl in morphine and the phenyl of phenylethyl in R31833, the O(51) and O(52) in Leu⁵-enkephalin corresponds to the O(3) in morphine and the O(25) in R31833, the O(4) in Leu⁵-enkephalin corresponds to the O(29) or O(27) in R31833. On the basis of these studies, it was inferred that these compounds have common feature in pharmacophore. They not only have the same way of action but also have common site of action in the receptor when they interact with the opiate receptor.