The study reports evidence for the heterodimerization of two fully functional opioid receptors, κ and δ, which results in a new receptor with distinct ligand-binding and functional properties. This heterodimer exhibits synergistic binding of highly selective agonists and enhances signal transduction. The κ-δ heterodimer is stable in various detergents and is destabilized by reducing agents, suggesting the involvement of disulfide bonds in its formation. The heterodimer also shows altered trafficking properties, with reduced internalization of δ-receptors when co-expressed with κ-receptors. The ligand-binding properties of the κ-δ heterodimer differ from those of individual κ or δ receptors, showing no significant affinity for either κ- or δ-selective agonists or antagonists, but a strong affinity for partially selective ligands. These properties are similar to those of the previously reported κ-2 receptor subtype. The study also shows that the κ-δ heterodimer binds selective agonists synergistically, leading to increased effector function, as evidenced by enhanced phosphorylation of mitogen-activated protein kinase (MAPK) and reduced intracellular cyclic AMP levels. The findings suggest that heterodimerization of opioid receptors could represent a novel mechanism that modulates their function and may provide a molecular basis for other receptor subtypes. The study provides biochemical and functional evidence for opioid-receptor heterodimerization, which is the first direct evidence for the heterodimerization of opioid-receptor types and for two fully functional GPCRs. The results highlight the importance of heterodimerization in receptor function and suggest that it could be a mechanism for activating receptors on the co-release of selective endogenous peptides. The study also indicates that heterodimerization of opioid receptors with other GPCRs could be a target for endogenous opioid peptides. The findings have significant implications for understanding how GPCR actions are regulated and for the development of therapeutic drugs.The study reports evidence for the heterodimerization of two fully functional opioid receptors, κ and δ, which results in a new receptor with distinct ligand-binding and functional properties. This heterodimer exhibits synergistic binding of highly selective agonists and enhances signal transduction. The κ-δ heterodimer is stable in various detergents and is destabilized by reducing agents, suggesting the involvement of disulfide bonds in its formation. The heterodimer also shows altered trafficking properties, with reduced internalization of δ-receptors when co-expressed with κ-receptors. The ligand-binding properties of the κ-δ heterodimer differ from those of individual κ or δ receptors, showing no significant affinity for either κ- or δ-selective agonists or antagonists, but a strong affinity for partially selective ligands. These properties are similar to those of the previously reported κ-2 receptor subtype. The study also shows that the κ-δ heterodimer binds selective agonists synergistically, leading to increased effector function, as evidenced by enhanced phosphorylation of mitogen-activated protein kinase (MAPK) and reduced intracellular cyclic AMP levels. The findings suggest that heterodimerization of opioid receptors could represent a novel mechanism that modulates their function and may provide a molecular basis for other receptor subtypes. The study provides biochemical and functional evidence for opioid-receptor heterodimerization, which is the first direct evidence for the heterodimerization of opioid-receptor types and for two fully functional GPCRs. The results highlight the importance of heterodimerization in receptor function and suggest that it could be a mechanism for activating receptors on the co-release of selective endogenous peptides. The study also indicates that heterodimerization of opioid receptors with other GPCRs could be a target for endogenous opioid peptides. The findings have significant implications for understanding how GPCR actions are regulated and for the development of therapeutic drugs.