Recently two tetrapeptide ligands that bind preferentially to the mu-opioid receptor were identified and named endomorphin-1 and endomorphin-2. We examined the ability of these peptides to stimulate G protein activation in human mu-opioid receptor transfected B82 fibroblasts as measured by [35S]GTPgammaS binding to cell membranes. Both endomorphin-1 and -2 act as partial agonists in this assay system compared with the mu-selective agonist [D-Ala2,N-Me-Phe4, Gly-ol5]enkephalin (DAMGO). In addition, endomorphins demonstrate efficacy similar to morphine. These findings demonstrate that endomorphin peptides have similar activity at the mu-opioid receptor as morphine and suggest that these peptides have the potential to modulate neuronal activity in vivo.