-Aminobutyric acid (GABA)-mediated transmission in the medial preoptic area (MPOA) of the hypothalamus plays an important role in functions such as sex steroid hormone dynamics and control of body temperature. The action of allopregnanolone, the primary metabolite of progesterone, on GABAergic transmission was investigated by employing patch clamp whole cell recording on acutely dissociated rat MPOA neurons with the functional connection of presynaptic terminals. Allopregnanolone enhanced spontaneous GABA release on the MPOA neurons and induced prolonged decay of miniature GABAergic-inhibitory postsynaptic currents (mIPSCs). The facilitation of GABA release from the presynaptic terminals by allopregnanolone disappeared in Ca²⁺-free extracellular solution. The presynaptic action of this neurosteroid was also blocked by bumetanide, a blocker of cation-Cl cotransporters, and by removal of extracellular Na⁺. The results suggest that allopregnanolone enhances GABAergic transmission at the MPOA neurons by pre- and postsynaptic mechanisms. The enhancement of GABA release by allopregnanolone might require a high Cl concentration in the presynaptic terminal maintained by Na⁺-dependent, bumetanide-sensitive mechanisms (e.g., Na⁺-K⁺-Cl cotransporter) and might be mediated by Ca²⁺ influx into presynaptic terminal.