Male Wistar rats were trained to discriminate either the opioid receptor like (ORL)-1 receptor agonist 3,3a,4,5,3,decan-4-one) or morphine from saline using a twochoice, food reinforced, operant procedure. Acquisition of Ro 64-6198 discrimination was relatively slow (mean trials to criterion 113 Ϯ 6), and a final 4 mg/kg dose (initial training dose 2 mg/kg) was required to establish appropriate stimulus control. In comparison, a separate group of rats attained a morphine (2 mg/kg) discrimination in 44 Ϯ 4 trials. In tests of substitution, Ro 64-6198 produced a dose-related generalization to its own cue (ED 50 of 1.1 mg/kg i.p.), yet only weakly generalized to the morphine cue (19% at 10 mg/kg i.p.). In contrast, morphine generalized completely to the morphine cue (ED 50 of 0.7 mg/kg s.c.), yet only partially generalized to the Ro 64-6198 cue (40% at 6 mg/kg s.c.). The opioid receptor agonist U50,488 [trans-3,4-(0.3-6 mg/kg s.c.) and the ␦ opioid receptor agonist SNC-803-6 mg/kg i.p.) failed to evoke significant generalization to either cue. The opioid receptor agonists codeine (0.3-20 mg/kg) and buprenorphine (0.01-1 mg/kg) completely generalized to the morphine cue, but only buprenorphine partially generalized to the Ro 64-6198 cue. Naloxone pretreatment completely blocked the morphine cue (ED 50 of 0.005 mg/kg s.c.), yet only weakly attenuated the Ro 64-6198 cue at 0.3 mg/kg. Finally, the selective ORL-1 antagonist J-113397 [1-[(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1, 3-dihydro-2H-benzimidazol-2-one] completely blocked the Ro 64-6198 cue at a dose (30 mg/kg i.p.) that had no effect against the morphine cue. The present studies demonstrate that rats may be trained to discriminate Ro 64-6198 from saline, and the pharmacological characteristics of this cue are most consistent with ORL-1 receptor activation.Opioid receptors are currently classified into four subclasses, the ␦ opioid peptide receptor, opioid peptide receptor, opioid peptide receptor, and the nociceptin/orphaninFQ peptide receptor (also termed ORL-1) subclass (Calo et al., 2000;Snyder and Pasternak, 2003). The ORL-1 receptor represents the most recent addition to this family, identified by various laboratories during efforts to clone subtypes to the , , or ␦ receptor subclasses (for recent reviews, see Calo et al., 2000;Mogil and Pasternak, 2001).The ORL-1 receptor has a pharmacology that is clearly distinct from the other opioid subclasses. For example, naloxone, a relatively nonselective antagonist at , , and ␦ receptors, has low affinity for the ORL-1 subtype (Calo et al., 2000;Mogil and Pasternak, 2001). Furthermore, nonpeptide ligands apparently selective for the ORL-1 receptor have been identified, notably the agonist Ro 64-6198 (Jenck et al., 2000), and the antagonist J-113397 (Ozaki et al., 2000). Ro 64-6198 seems to function as a full agonist, equivalent to the likely endogenous agonist orphaninFQ/nociceptin (OFQ/N; Calo et al., 2000) in cell-based systems expressing the human ORL-1 receptor, and with at le...