Oestradiol-17ß, in its positive feedback mode, as well as stimulating the release of luteinizing hormone-releasing hormone. (LHRH), also stimulates LHRH mRNA synthesis in medial preoptic neurons. Our aim was to determine whether the action of oestradiol might be mediated in part by inhibiting arcuate proopiomelanocortin (POMC) neurons which terminate on LHRH neurons. Female rats were ovariectomized at about 1000 h of dioestrus, given a subcutaneous injection of oil or oestradiol benzoate and killed at 1600 h the same day (presumptive dioestrus), presumptive pro-oestrus and oestrus and at 1000 h of presumptive pro-oestrus. In situ hybridization of brain sections was carried out with a [(35) S]UTP-labelled riboprobe containing 538 bases of a cDNA identical to exon 3 of the POMC gene, which includes the ß-endorphin coding domain. The amount of POMC mRNA in cells of the anterior arcuate nucleus, as assessed by grain counts, was significantly decreased in oestradiol benzoate- compared with oil-treated animals at 1600 h of pro-oestrus, but at none of the other times studied. This finding, together with other data, suggests that the positive feedback stimulation of LHRH mRNA synthesis by oestradiol-17ß may be due, in part, to the disinhibition of LHRH neurons consequent on inhibition of POMC mRNA biosynthesis in arcuate neurons.