Abstract: Opioid receptors form functional heterodimers with each other and with other G-protein coupled receptors, such as dopamine receptors, adrenergic receptors, or chemokine receptors. These receptors can be exploited for high throughput screening of compounds to identify heterodimer opioid receptor modulators (agonists and antagonists). The invention also relates to identification of novel heterodimer receptor ligands and synergistic compositions, which can provide strategies for analgesia, narcotic addiction, hypertension, HIV infection, and immune system function.