The treatment of pain continues to be the subject of considerable pharmaceutical and clinical research. R2 Adrenoceptor agonists are intriguing because they produce clinically effective analgesia that is devoid of opioid-related side effects. The R2 adrenergic agonist clonidine (1) is prescribed for the epidural treatment of severe pain, and the R2 agonist dexmedetomidine (2) is used in veterinary practice and for humans in several countries outside the United States. We have recently reported the potent R2 adrenergic receptor affinity and analgesic activity associated with (imidazomethyl)oxazoles and -thiazoles, such as 3 (RWJ-37210), and investigated a large number of (imidazomethyl)thiophenes, such as 4, that have a similar biological profile. Analyzing the preferred conformations of 4 by molecular modeling revealed significant torsional freedom for rotation of the two single bonds between the imidazole and thiophene. To restrict the rotation of this molecule, we extended and combined the 2-ethyl substituent with the bridging carbon atom to obtain the (imidazo)thianaphthene chemical series (e.g. 8 and 9, Scheme 1). These compounds are potent R2 adrenergic receptor ligands, and certain members of the series are very active in animal models of antinociception. 4-(4-Imidazo)-6,7-dihydrothianaphthene (14) has a favorable biological profile in vivo and is a prototype for this class of analgesic agents. All of the compounds tested showed excellent receptor affinity for the R2D adrenergic receptor subtype. Saturated derivatives 9 (0.35 nM Ki) and 15 (0.56 nM Ki) were ca. 3-10 times more potent than their singly unsaturated counterparts 8 and 14. Fully aromatic thianaphthene 19 also had good affinity (2.6 nM Ki). Surprisingly, alkene 22 was exquisitely active at the R2D adrenergic receptor, showing a very high affinity of 0.0086 nM Ki. The (imidazo)thianaphthenes are a new class of R2 adrenergic agents that display potent receptor binding affinity and are active in animal models of analgesia. 4-(4-Imidazo)-1,3-dimethyl-6,7-dihydrothianaphthene (22) is an exceedingly potent ligand for the R2D adrenergic receptor (Ki = 0.0086 nM) and is >10 000 fold selective relative to the R1 adrenergic receptor. This compound is more potent than dexmedetomidine at R2D and is a useful pharmacological tool drug for studies involving the R2D adrenergic receptor.