Nociceptin (NC), a series of NC fragments, naloxone as well as the pseudopeptide [Phe1psi(CH2-NH)Gly2]NC(1-13)NH2 ([F/G]NC(1-13)NH2) were used to characterize NC receptors in peripheral isolated organs and in vivo. Experiments on isolated organs were performed in the mouse (mVD) and rat (rVD) vas deferens (noradrenergic nerve terminals), in the guinea pig ileum (gpI; cholinergic nerves) and in the renal pelvis (gpRP; sensory nerves), and, in vivo, by measuring the blood pressure (BP) and heart rate (HR) in anaesthetised rats. NC, NCNH2 and NC(1-13)NH2 acted as full agonists with similar affinities, while shorter fragments (e.g. NC(1-12)NH2, NC(1-9)NH2, NC(1-5)NH2) were much weaker or inactive. The inhibitory effects of NC were not modified by naloxone. [F/G]NC(1-13)NH2 acted as an antagonist with similar pA2-values (6.75 mVD, 6.83 rVD, 7.26 gpI) in the three species. In addition, it blocked NC actions in the rat in vivo. Linear Schild plots with slopes near to unity indicated that [F/G]NC(1-13)NH2 is a competitive antagonist, specific for NC receptors both in vitro (since it was inactive on opioid receptors) and in vivo (since it was inactive against carbachol). [F/G]NC(1-13)NH2 showed a residual agonistic activity in vitro (alpha = 0.2-0.3 in the rVD and gpI) and especially in vivo (alpha = 0.4 BP, 0.2 HR). These pharmacological data indicate that NC and related peptides exert their inhibitory effects in peripheral organs of various species by activating the same receptor type. Moreover, [F/G]NC(1-13)NH2 appears to be a useful tool for receptor characterization and classification.