Selective PCR amplification of human and mouse genomic DNAs with oligonucleotides encoding highly conserved regions of the delta-opioid and somatostatin receptors generated a human DNA probe (hOP01, 761 bp) and its murine counterpart (mOP86, 447 bp). hOP01 was used to screen a cDNA library from human brainstem. A clone (named hORL1) was isolated, sequenced and found to encode a protein of 370 amino acids whose primary structure displays the seven putative membrane-spanning domains of a G protein-coupled membrane receptor. The hORL1 receptor is most closely related to opioid receptors not only on structural (sequence) but also on functional grounds: hORL1 is 49-50% identical to the murine mu-, delta- and kappa-opioid receptors and, in CHO-K1 cells stably transfected with a pRc/CMV:hORL1 construct, ORL1 mediates inhibition of adenylyl cyclase by etorphine, a 'universal' (nonselective) opiate agonist. Yet, hORL1 appears not to be a typical opioid receptor. Neither is it a somatostatin or sigma (N-allylnormetazocine) receptor. mRNAs hybridizing with synthetic oligonucleotides complementary to mOP86 are present in many regions of the mouse brain and spinal cord, particularly in limbic (amygdala, hippocampus, septum, habenula, ...) and hypothalamic structures. We conclude that the hORL1 receptor is a new member of the opioid receptor family with a potential role in modulating a number of brain functions, including instinctive behaviours and emotions.