1. The ORL1 receptor and its endogenous peptide nociceptin or orphanin FQ (N/OFQ) are novel members of the opioid receptor family. Despite the homology displayed between the opioid and ORL1 systems the ORL1 receptor and nociceptin display anti-opioid properties. Here issues of opioid-ORLl receptor co-operativity have been addressed using ORL1 and μ/δ/κ triple knockout mice. ORL1 and opioid receptors have been mapped autoradiographically in the brains of knockout mice. There was a significant region specific increase in ORL1 receptor expression in the brains of μ/δ/κ triple knockout mice. Similarly an up regulation of μ-and κ- receptors is evident in the ORL1 receptor knockouts suggesting a region specific interdependence of the two systems. 2. Since the discovery of the opioid peptides and their receptors there has been little evidence for which peptide primarily acts at which opioid receptor sub-type, μ-δ κ-and ORL1 receptors were mapped in N/OFQ, dynorphin, enkephalin and dynorphin/enkephalin double knockout mice. A region-specific up-regulation of ORL1 receptors was observed in N/OFQ knockout mice suggesting a tonic role for the ORL1 receptor in functions mediated by ventral medial hypothalamic nucleus, hypothalamus and suprachiasmatic nucleus where the greatest up regulation of receptors was seen. Similarly a region specific up regulation of κ- receptors was observed in dynorphin knockouts and of μ- and δ- receptors in enkephalin knockouts indicating that the κ- receptor is the primary target for dynorphin and μ- and δ- receptors are the primary target for enkephalin. In addition an up regulation of μ - and δ- receptors was observed in dynorphin knockouts suggesting a degree of cross reactivity for dynorphin at these receptors. 3. Receptor knockout mice have also been used here to address issues of receptor heterogeneity. [3H]Naloxone binding was completely lost in μ/δ/κ triple knockout mice providing assurance that there are no further classical opioid receptors encoded for by other genes. In addition [3H]bremazocine binding in μ/δ/κ triple knockout mice have been used in association with findings from μ/κ double knockout mice to confirm that there are no further κ-receptor subtypes derived from other genes and that the higher levels of benzomorphan binding to K-receptors compared with arylcetamide compounds can be attributed to incomplete suppresion of μ- and δ- sites by cold competing ligands.