Histamine is capable of modulating CNS arousal state by regulating neuronal excitability. In current study, histamine action in ventromedial hypothalamus (VMH), its related ionic mechanisms and a possible interaction with estrogen were investigated using whole-cell patch-clamp recording in brain slices from ovariectomized female mice. Under current clamp condition, bath application of histamine (20 µM) caused membrane depolarization, associated with an increased membrane resistance. In some cells, the depolarization was accompanied by action potential firing. Histamine application also significantly reduced the latency of action potential evoked by current steps. Histamine-induced depolarization was not affected by either tetrodotoxin or Cd²⁺. However after blocking K⁺ channels with tetraethylammonium, 4-aminopyridine and Cs⁺, depolarization was significantly decreased. Under voltage clamp condition, histamine-induced depolarization was associated with an inward current. The current voltage relationship revealed that this inward current reversed near E_K. The histamine effect was mimicked by an histamine receptor 1 (H₁) agonist, but not an histamine receptor 2 (H₂) agonist. H1 antagonist, but not H₂ antagonist, abolished histamine responses. When ovariectomized mice were treated with estradiol benzoate (E2), histamine-induced depolarization was significantly enhanced with increased percentage of cells showing action potential firing. These results suggest that histamine depolarized VMH neurons by attenuating K⁺ leaking current and this effect was mediated via H₁ receptor. E2 facilitated histamine-induced excitation of VMH neurons; this may present a potential mechanism by which estrogens modulate the impact of general CNS arousal on sexual arousal-related neuronal group.