A review of the binding studies performed on the receptor (ORL) for Orphanin FQ/Nociceptin is presented. Binding studies have been conducted using a variety of receptor sources: cell lines expressing the cloned receptor, cell lines endogenously expressing the receptor, and brain and other tissue from several different species. Binding studies of opioids, new ligands and antagonists at the ORL receptor are briefly discussed. Saturation, competition and binding kinetic experiments, and the effects of buffer composition are reviewed. There are numerous instances of conflicting data in published reports on OFQ; the basis for these disparities is as yet undetermined. This review endeavors to compile the results and conditions employed in binding studies as an aid to current and new researchers in this field. In an attempt to explain binding disparities, we have determined that Orphanin/Nociceptin binds to glass fiber filtermats in a 'specific' manner; these new data are presented. Copyright (C) 2000 Elsevier Science Inc.