The effect of Al cell group electrical stimulation on the simultaneous release of endogenous noradrenaline (NA) and γ-aminobutyric acid (GABA) from the medial preoptic area (MPOA) was monitored with microdialysis. In ovariectomised (OVX) rats a 15-min period of Al stimulation induced an immediate increase in both NA and GABA release in the MPOA. Electrical stimulation lateral to the A1 region did not alter NA or GABA release. The addition of the α-adrenergic antagonist phenoxybenzamine to the perfusion medium resulted in a significant increase in basal NA levels, and electrical stimulation during this period further increased NA release while GABA levels were not significantly altered throughout. The effect of oestrogen on this pathway was examined in animals at a time of oestrogen-negative feedback on luteinising hormone (LH) secretion (OVX-EBn) and prior to the expected oestrogen-induced LH surge (OVX-EBp). Activation of Al neurones in OVX-EBn rats resulted in NA and GABA increases in the MPOA similar to that observed with OVX rats. In OVX-EBp animals, basal GABA levels were found to be significantly higher compared with OVX rats but NA release induced by Al stimulation had no effect on GABA levels. Depolarisation of the MPOA by increasing the potassium ion concentration of the perfusion medium evoked significantly greater GABA release from OVX-EBp rats compared with the OVX and OVX-EBn animals. Potassium-stimulated NA release was not significantly altered by oestrogen administration. These results demonstrate an excitatory α-adrenergic mediated noradrenergic input to GABA neurones in the MPOA. We provide evidence for oestrogen modulation of this pathway at the time of the LH surge and suggest that GABA neurones in the MPOA may mediate some of the known effects of NA on LH secreti