An endorphin is any of a group of hormone-like substances in the brain and other tissues that have pain-killing and tranquilizing properties like those of morphine and other opiates. Endorphins are peptides or polypeptides. Several of them are associated with the pituitary gland, while others occur in the placenta, adrenal gland, and pancreas. The existence of these substances, which are also known as endogenous opiates, first became apparent in the course of research into opiate drugs such as heroin and morphine. See also analgesic.