The possible role of prostaglandins (PGs) in the hypothalamic control of ovulation has been investigated by studying the effects of indomethacin (ID) and PGs on the release of luteinizing hormone-releasing hormone (LHRH) from hypothalamic synaptosomes. Female rats were chronically pretreated with ID on diestrus 2 (D<sub>2</sub>) and proestrus (P), or on diestrus 1 (D<sub>1</sub>) and D<sub>2</sub>. Basal LHRH release from hypothalamic synaptosomes prepared from rats pretreated on D<sub>2</sub>/P was reduced to 48% after a 20-min incubation period, compared with sham-treated animals. ID pretreatment on D<sub>1</sub>/D<sub>2</sub> caused a similar but less marked reduction in LHRH release (to 66% of controls at 20 min). Extraction of the LHRH remaining within the synaptosomal pellet showed that ID pretreatment also reduced the synaptosome content of LHRH. (15S)-15-methyl PGE<sub>2</sub> (15-E<sub>2</sub>) (10<sup>-4</sup> and 10<sup>–6</sup> M) added to the incubating medium stimulated the release of LHRH by approximately 40%, but this effect was only observed in synaptosomes prepared from ID-treated rats. PGE<sub>2</sub> (10<sup>–4</sup> and 10<sup>–6</sup> M) had no significant effect in either sham-treated or ID-treated groups.