This investigation used an in vitro hypothalamic brain slice preparation and whole cell and perforated-patch recording to examine the response of magnocellular neurons in hypothalamic paraventricular nucleus (PVN) to bath applications of vasopressin (VP; 100-500 nM). In 22/38 cells, responses were characterized by an increase in the frequency of bicuculline-sensitive inhibitory postsynaptic potentials or currents with no detectable influence on excitatory postsynaptic events. Perforated-patch recordings confirmed that VP did not have an effect on intrinsic membrane properties of magnocellular PVN neurons (n = 17). Analysis of intrinsic membrane properties obtained with perforated-patch recording (n = 23) demonstrated that all of nine VP-sensitive neurons showed a rebound depolarization after transient membrane hyperpolarization from rest. By contrast, 12/14 nonresponding neurons displayed a delayed return to resting membrane potentials. Recordings of reversed inhibitory postsynaptic currents with chloride-loaded electrodes showed that responses to VP persisted in media containing glutamate receptor antagonists but were abolished in the presence of tetrodotoxin. In addition, responses were mimicked by vasotocin [Phe(2), Orn(8)], a selective V(1a) receptor agonist, and blocked by [beta-Mercapto-beta, beta-cyclopentamethylenepropionyl(1),O-Me-Tyr(2), Arg(8)]-VP (Manning compound), a V(1a)/OT receptor antagonist. Neither [deamino-Cys(1),Val(4),D-Arg(8)]-VP, a selective V(2) receptor agonist, nor oxytocin were effective. Collectively, the results imply that VP acts at V(1a) receptors to excite GABAergic neurons that are presynaptic to a population of magnocellular PVN neurons the identity of which features a unique rebound depolarization. Endogenous sources of VP may be VP-synthesizing neurons in suprachiasmatic nucleus, known to project toward the perinuclear regions of PVN, and/or the magnocellular neurons within PVN.