The lack of availability of a selective, highly potent, competitive antagonist for the nociceptin receptor (OP4) devoid of residual agonistic activity has hampered studies in this area. We report here the in vitro pharmacological properties of the novel non-peptide OP4 antagonist, J-113397, which was recently discovered by Banyu Pharmaceutical investigators. The compound was synthesized as a racemic mixture in our laboratories. J-113397 was shown to antagonize (pA2 7.52) the nociceptin-induced inhibition of cAMP formation in cells expressing the recombinant human OP4 receptor (CHOhOP4) and to displace [125I]Tyr14nociceptin from CHOhOP4 membranes with a pKi of 8.56. It also competitively antagonized the contractile actions of nociceptin in the mouse colon (pA2 8.07) and the inhibitory effect of nociceptin in electrically stimulated preparations such as the mouse vas deferens (pA2 7.85), the guinea pig ileum (7.75), and the rat vas deferens (7.77). At high concentrations (10 microM), the compound was devoid of agonist activity in the mouse vas deferens and CHOhOP4, while it contracted the mouse colon and increased the twitch response of the rat vas deferens, and produced a naloxone-sensitive inhibition of the electrically evoked twitches in the guinea pig ileum. pA2 values for the new antagonist against deltorphin I in the mouse vas deferens (OP1 receptors), or against dermorphin in the guinea pig ileum (OP3 receptors), etorphine in the rat vas deferens (OP receptors), U69593 in the rabbit vas deferens (OP2 receptors) and endomorphin 1 in the mouse colon (OP3 receptors) were lower than 6. Taken together, these data indicate that J-113397 is a high-affinity, selective and competitive antagonist of the OP4 receptor; this novel pharmacological tool will be of great value in studies directed at evaluating the physiological roles of the nociceptin/OP4 system.