Prostaglandins (PGs), especially PGE(2), are involved in the hypothalamic control of gonadotrophin-releasing hormone (GnRH) release, acting at least in part on the terminal of GnRH axons in the median eminence. The present study aimed: (i) to clarify the role of PG(s) in regulating GnRH cell function at the level of the perikarya in the preoptic area; (ii) to determine the cyclooxygenase (COX) isozyme responsible for producing PG(s) that regulates GnRH perikarya; and (iii) to identify cell types that contain the responsible COX isozyme in female rats. A surge of luteinising hormone (LH) secretion was induced by oestrogen and progesterone in ovariectomised rats. Treatment of the rat before the LH surge with indomethacin, a nonselective COX inhibitor, or NS-398, a selective COX-2 inhibitor, did not interfere with the surge. However, treatment with indomethacin or flurbiprofen, a selective COX-1 inhibitor, significantly reduced the number of GnRH-immunoreactive cells in the preoptic area at the time of peak LH secretion during the surge. NS-398 did not affect the GnRH immunoreactivity. Double-labelled immunofluorescent histochemistry revealed COX-1 immunoreactivity in the vicinity of, but not within, GnRH containing neurones in the preoptic area. COX-2 immunoreactivity was not found in the same area. The COX-1 immunoreactivity was almost entirely localised in microglia in the preoptic area, but not in neurones or astrocytes. These results suggest that microglia in the preoptic area containing COX-1 are responsible for producing PG(s), which, in turn, facilitates the accumulation of GnRH during the gonadotrophin surge in female rats.