Push-pull cannulae were implanted into the mediobasal hypothalamus of ovariectomized (ovx) rats. After recovery animals were treated with estradiolbenzoate (E<sub>2</sub>B) or oil and they were perfused 3 days later. Only the E<sub>2</sub>B-treated animals which exhibited prolactin surges in the afternoon without concomitant LH surges were used in this study. In ovx animals hypothalamic GABA release, measured in 5-min intervals, was pulsatile, with pulses occurring every 37 min. This pattern was profoundly affected by E<sub>2</sub>B treatment: the pulse frequency was significantly reduced to 1 pulse every 117 min in steroid-treated rats. No differences in overall mean GABA release rates and pulse amplitudes were observed in ovx vs. E<sub>2</sub>B-treated rats. Our earlier demonstration of the existence of a large number of estrogen-receptive, GABAergic neurons in the MBH of rats is suggestive that these neurons change their secretory pattern in response to estrogen treatment. Estrogen-induced prolactin surges were accompanied by increased hypothalamic NE release. Concomitant changes in DA or E release rates were not demonstrable since catecholamine concentrations were too low to be reliable. However, the daily overall release rates of these two catecholamines were lower in E<sub>2</sub>B-treated rats compared to ovx animals. It is concluded that the positive feedback action of estradiol on prolactin release involves a stimulatory noradrenergic mechanism and may also involve estrogen-receptive, GABAergic neurons. The alterations of pulse frequency of these GABAergic neurons appear to define the particular signal transmitted to the endocrine target cells, whereas the total amount of GABA released over a longer period of time does not seem to be of importance for the estrogen-dependent function modulated by this GABAergic system. Whether this estrogen-dependent function is regulatory to prolactin and/or gonadotropin secretion remains to be determined.