The orphanin-FQ/nociceptin (OFQ/N) receptor (previously, ORL1, LC132) has been shown to be coupled to an inwardly rectifying K+ conductance in several neuronal populations. Although OFQ/N receptor mRNA is densely expressed in the supraoptic nucleus (SON), little is known about its coupling to effector system(s). The present study examined the effects of OFQ/N on guinea pig magnocellular neurons within the SON using intracellular recording from hypothalamic slices. In the presence of tetrodotoxin, OFQ/N hyperpolarized 48 of 48 SON magnocellular neurons, 24 of which were subsequently identified by immunocytochemistry as arginine vasopressin positive (AVP+). Nineteen of the 48 SON neurons, including 7 which were AVP+, responded to OFQ/N with an outward current that reversed at the K+ equilibrium potential (EK+) and a decrease in slope resistance consistent with the activation of an inwardly rectifying K+ channel. In 4 of these neurons, BaCl2 significantly attenuated both the hyperpolarization and the decrease in slope resistance induced by OFQ/N. Twenty-one SON neurons, 13 of which were AVP+, responded to OFQ/N with an increase in slope resistance which did not reverse at EK+. An additional 5 neurons (2 were AVP+) were treated with the gap junction blocking agent carbenoxolone (CARB). CARB induced a small hyperpolarization, increased slope resistance and significantly reduced the subsequent OFQ/N-induced hyperpolarization. However, when the CARB and CARB plus OFQ/N hyperpolarizations were summed in these 5 cells, they were no different than the OFQ/N hyperpolarization alone. The effect of two putative OFQ/N receptor antagonists was also evaluated. The kappa3-opioid antagonist naloxone benzoylhydrazone was without effect (n = 3), and the 13-amino-acid [Phe1Psi(CH2-NH)Gly2]OFQ/N(1-13)NH2 OFQ/N analog produced a small hyperpolarization on its own in addition to partially antagonizing the effects of OFQ/N (n = 3). Taken together, these results suggest that OFQ/N acts upon SON neurons through two mechanisms, one which hyperpolarizes the neuron by activating an inwardly rectifying K+ conductance, and another which may increase membrane resistance by closing the low-resistance gap junctions.