The role of GABA neurons in the control of the surge of LH secretion was investigated by examining whether GABAA receptor antagonist bicuculline (BIC) infusion prior to the LH surge could advance the timing of the proestrous surge in the rat. The effect of the opioid receptor antagonist naloxone (NAL) was also examined for comparison. Female rats in proestrus were iv infused with NAL (2 or 5 mg/hr) or BIC (50 mg/kg/hr) for a 3-hr (1000-1300 hr) period during which blood samples were collected at 6-min intervals through an intraatrial cannula in freely moving rats. The animals that received infusion were bled again over a 7-hr period (1400-2000 hr) at 1-hr intervals. NAL infusions at 2 mg/hr induced an increase in pulsatile LH secretion, but did not affect the timing, magnitude, and duration of the LH surge. NAL infusions at 5 mg/hr not only induced a greater increase in pulsatile LH secretion but also resulted in a pronounced LH surge prematurely. In contrast, during BIC infusion, no significant changes in LH secretion were seen until 1200 hr, but afterward a rapid and sharp rise in LH secretion occurred, suggesting the premature LH surge. These results are consistent with the hypothesis that the tonic LH secretion is sensitive to NAL, but not to BIC, whereas the surge of LH secretion is sensitive to barbiturates that have been known as the activator of GABAA receptor complex. Therefore, we suggest that there is a LHRH surge generator, distinct from the pulse generator, in the brain, and it has GABA neurons in its circuitry.