Body fluid balance requires the release of arginine vasopressin (AVP) from the neurohypophysis. The hypothalamic supraoptic nucleus (SON) is one of the major sites for the synthesis of AVP, and secretion of AVP is controlled by the electrical activities of magnocellular neurosecretory cells (MNCs), which in turn are regulated by neuronal excitatory glutamatergic and inhibitory GABAergic inputs and humoral factors such as plasma osmolality. Previous studies have shown that brain-derived neurotrophic factor (BDNF) mRNA was increased by osmotic stress in the rat SON. In the present study, the effects of BDNF on excitatory and inhibitory synaptic inputs were examined in the MNCs of rat SON, using the whole-cell patch-clamp technique in in vitro brain slice preparations. BDNF application caused a significant reduction in the frequency and amplitude of the spontaneous inhibitory postsynaptic currents of the MNCs without affecting the spontaneous excitatory postsynaptic currents. Next, whole-cell patch-clamp recordings from dissociated SON MNCs expressing AVP-enhanced green fluorescent protein (eGFP) transgene revealed that the amplitude of GABA-induced currents were significantly smaller after BDNF treatment. Moreover, multi-cell reverse transcriptase-polymerase chain reaction (RT-PCR) experiments revealed the expression of TrkB mRNA in AVP-eGFP neurons. These results suggest that BDNF in the rat SON may decrease the postsynaptic GABAergic activity and may be involved in the regulatory mechanisms of body fluid homeostasis.