Orphanin FQ was identified and purified from porcine brain tissue as a ligand for an orphan heterotrimeric GTP-inding protein (G protein)-coupled receptor (LC132) that is similar in sequence to opioid receptors. Orphanin FQ has a primary structure reminiscent of that of opioid peptides. Nanomolar concentrations of orphanin FQ have inhibited forskolin-stimulated adenylyl cyclase activity in cells transfected with LC132. This inhibitory activity was not affected by the addition of opioid ligands, nor did the peptide activate opioid receptors. Orphanin FQ bound to its receptor in a saturable manner and with high affinity.