Peptides have been identified in mammalian brain that are considered to be endogenous agonists for the delta (enkephalins) and kappa (dynorphins) opiate receptors, but none has been found to have any preference for the mu receptor. Because morphine and other compounds that are clinically useful and open to abuse act primarily at the mu receptor, it could be important to identify endogenous peptides specific for this site. Here we report the discovery and isolation from brain of such a peptide, endomorphin-1 (Tyr-Pro-Trp-Phe-NH2), which has a high affinity (Ki = 360 pM) and selectivity (4,000- and 15,000-fold preference over the delta and kappa receptors) for the mu receptor. This peptide is more effective than the mu-selective analogue DAMGO in vitro and it produces potent and prolonged analgesia in mice. A second peptide, endomorphin-2 (Tyr-Pro-Phe-Phe-NH2), which differs by one amino acid, was also isolated. The new peptides have the highest specificity and affinity for the mu receptor of any endogenous substance so far described and they may be natural ligands for this receptor.