Glycyl-L-glutamine is a C-terminal dipeptide fragment of β-endorphin . It decreases reperfusion damage and increases the time to ischemic contracture in isolated rat hearts in a model of cardiac ischemia-reperfusion injury using eight- and 36-week old rats when used at a concentration of 5 mM. Glycyl-L-glutamine (1-100 nmol) inhibits morphine-induced respiratory depression with no effect on morphine-induced analgesia in rats. It also inhibits acquisition of place preference, delays the development of tolerance, and reduces withdrawal intensity in morphine-conditioned rats.