Description_x000D_ Biochem/physiol Actions_x000D_ Ro 64-6198 is a brain-penetrant, nonpeptidic, high-affinity (Ki = 0.389 nM against 0.1 nM OFQ for binding human ORL1) nociceptin/orphanin FQ (N/OFQ) receptor ORL-1 (KOR-3, NOP) agonist (EC50 = 38.9 nM by GTPγS binding assay; IC50 = 32.4 nM against forskolin-stimulated cellular cAMP accumulation) with >100-fold selectivity over other opioid receptor family members (Ki = 46.8 nM/μ, 89.1 nM/k, 1.38 μM/δ) and no significant affinity toward 48 other receptors and ion channels (IC50 >1 μM). Ro 64-6198 elicits ORL1-dependent anxiolytic-like effects in several rat models of spontaneous and conditioned anxiety states (0.3-3.2 mg/kg i.p) without antipanic, anticonvulsant, sedative, amnestic activity, or signs of tolerance to its anxiolytic-like effects following chronical dosing.