Many researchers believe that strenuous exercise releases endorphins into the blood stream. Others agree that endorphins are released during orgasm, as well as during laughter. Endorphins are a group of substances formed within the body that naturally relieve pain. They have a similar chemical structure to morphine. In addition to their analgesic affect, endorphins are thought to be involved in controlling the body's response to stress, regulating contractions of the intestinal wall, and determining mood.