Arylacetamide analgesics that stimulate kappa opioid receptors in the central nervous system mediate dysphoria and psychosis as well as analgesia. However, the naturally occurring peptide agonist, dynorphin A, is analgesic in the absence of dysphoria and psychosis, indicating that the therapeutic effects of kappa opioid agonists may be separated from their side effects. As part of our effort to discover kappa opioid receptor analgesics lacking side effects, we designed and constructed two mu/kappa chimeric receptors, composed primarily of amino acid residues derived from the mu opioid receptor, that were expected to bind dynorphin A with high affinity. In one, extracellular loop 2 and transmembrane domain 4 were derived from the kappa opioid receptor and in the other, only extracellular loop 2 was derived from the kappa opioid receptor. Most competitors of [(3)H]diprenorphine binding from a variety of structural classes bound to the chimeras with affinities similar to those with which they bound to the mu opioid receptor. In contrast, dynorphin A analogs bound to the chimeras with affinities similar to those with which they bound to the kappa opioid receptor. Pharmacological characterization of [(35)S]GTPgammaS binding mediated by the chimera with extracellular loop 2 derived from the kappa opioid receptor showed that it behaved as if it were mu opioid receptor with high affinity for dynorphin A analogs. These chimeras may be useful in identifying novel kappa receptor agonists that bind to the second extracellular loop of the receptor and share the desirable therapeutic profile of dynorphin A.