Endomorphin-1 (EM-1) is a recently isolated endogenous peptide having potent analgesic activity and high affinity and selectivity for the mu-opioid receptor. Endomorphin-1 activates G proteins and inhibits adenylyl cyclase activity in membrane preparations from cells stably expressing the mu opioid receptor, an effect reversed by the mu receptor antagonist CTAP (D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2). However, endomorphin-1 appears to have no influence on cells stably expressing the delta opioid receptor.