Opioid receptor-like protein ORL1, the receptor for the neuropeptide nociceptin (also named orphanin FQ), has two alternatively spliced isoforms in the rat. This alternative splicing event is generated by retaining of intron 3, 81 bases in length, in the mRNA region encoding the second extracellular loop of ORL1. A full-length rat ORL1 receptor has 367 amino acid residues. However, as revealed by sequencing of rat ORL1 genomic DNA and cDNA, the insertion of the unspliced intron 3 brings in an in-frame stop codon and, therefore, creates a truncated open-reading frame encoding only the N-terminal half of ORL1 (from the N-terminus to an alternate extracellular tail C-terminal to the fourth transmembrane domain). The two alternatively spliced transcripts are differentially expressed in tissues. In transfected mammalian cells, the full-length ORL1 displays high-affinity and selective binding for nociceptin, and inhibits the production of cyclic AMP. In contrast, the truncated ORL1 binds nociceptin and other opioid peptides very poorly and non-selectively (affinity in micromolar range), and it does not mediate any inhibitory effects on cyclic AMP production. Apparently, this truncated ORL1 does not function as a receptor for nociceptin or other ligands tested. Such alternative splicing to create a truncated ORL1 receptor might be an endogenous mechanism to negatively regulate nociceptin/ORL1 functions.