Neurosecretion of hypothalamic GnRH was monitored during the LH surge in ovariecto-mized goats given estradiol using microdialysis sampling. The microdialysis probe with a polycarbon-ate dialysis membrane was stereotaxically implanted in the median eminence, and was perfused with the artificial cerebrospinal fluid at a flow rate of 5μl/min under conscious and unrestrained conditions. The LH surge was induced by means of an intravenous infusion of estradiol (3 μg/h, i.v.) for 16 h and hourly fractions of microdialysis perfusate were collected for GnRH measurement. The perfusate GnRH showed a marked increase to peak levels of 25.3±6.9 μg/ml over basal levels of 2.6±1.1μg/ml at the onset of the LH surge, and then gradually decreased so that the GnRH surge lasted longer than the LH surge. These results suggest that the LH surge is induced by a surge release of GnRH from the hypothalamus but ends despite a continued supply of GnRH.