The aim of this study was to investigate effects of oxytocin (OT) on electrical neuronal activities in rat subfornical organ (SFO) and compare its action with the well-described excitatory effects of blood-borne angiotensin II (ANG II) on the same SFO neurons. With the use of extracellular recordings from spontaneously active neurons in slice preparations of the SFO of male rats, 11.7% of tested neurons (n = 206) were excited and 9.7% were inhibited by superfusion with 10(-6) M OT. Both excitatory and inhibitory effects of OT were dose dependent with similar threshold concentrations and were blocked by a specific OT-receptor antagonist but not by a vasopressin receptor antagonist. Blocking synaptic transmission with low calcium medium suppressed only inhibitory effects of OT. All but one of the OT-sensitive neurons were also excited by superfusion with ANG II at a concentration much lower than required for OT, suggesting that synaptically released OT rather than blood-borne OT alters the activity of SFO neurons in vivo. The results support the hypothesis that neurally released OT may modulate SFO-mediated functions by acting on OT-sensitive neurons.