[Phe1psi(CH2-NH)Gly2]NC(1-13)NH2 has been tested in the electrically stimulated guinea pig ileum and mouse vas deferens, two nociceptin sensitive preparations. The new compound showed per se little or no effect in the two tissues, but it displaced to the right the concentration-response curves of nociceptin in a concentration-dependent manner. Schild analyses of the data indicated a competitive type of antagonism and pA2 values of 7.02 and 6.75 in the guinea-pig ileum and the mouse vas deferens, respectively. At 10 microM [Phe1psi(CH2-NH)Gly2]NC(1-13)NH2 does not modify either the inhibitory effect of deltorphin I (the selective delta opioid receptor agonist) in the mouse vas deferens or that of dermorphine (the selective mu opioid receptor agonist) in the guinea-pig ileum. The present findings indicate that [Phe1psi(CH2-NH)Gly2]NC(1-13)NH2 is a selective antagonist of the nociceptin receptor.