The subfornical organ is one of a specialized group of CNS structures devoid of a significant blood-brain barrier, collectively known as the circumventricular organs. While peptides are normally excluded from access to most regions of the CNS, the subfornical organ contains neurons with a high density of receptors for many circulating peptides, including vasopressin. There is a well-established role for the subfornical organ in stimulating the release of vasopressin, and recent evidence suggests that it may also play an important role in mediating the negative feedback actions of vasopressin. The aim of this study was to determine the direct effects of vasopressin on subfornical organ neurons through patch-clamp studies in a dissociated subfornical organ preparation. In current-clamp studies, bath application of 10 nM vasopressin caused depolarizations in 61%, hyperpolarizations in 11%, and no significant change in membrane potential in 28% of neurons tested. We then sought to determine the specific ion channels involved in regulating the vasopressin-induced excitability of subfornical organ neurons through voltage-clamp studies. Vasopressin (10 nM) decreased the peak outward current at +40 mV by 50% (n=7), which was blocked by pretreatment with a V1 receptor antagonist (n=5). Based on these findings, we carried out a systematic characterization of two subformical organ K+ channels, the delayed rectifier (I(K)) and the transient outward current (I(A)). Through voltage isolation of I(K), we found that vasopressin inhibited the steady-state current, by 33+/-7% (n=9). Vasopressin also inhibited the peak I(A) by 27+/-5% (n=8). These data provide the first evidence of a role for K+ channels in mediating the excitatory effects of vasopressin on subfornical organ neurons. The exact physiological roles and sources of vasopressin which may act on subfornical organ neurons are not completely understood at present.