The preovulatory luteinizing hormone (LH) surge in female rats is dependent upon signals from the suprachiasmatic nucleus (SCN), the site of a dominant circadian pacemaker. Various lines of evidence indicate that arginine-vasopressin (AVP)-containing projections from the SCN to the preoptic area (POA) contribute to the production of the surge of LH-releasing hormone (LHRH). These actions may be mediated by V(1a) because the transcript for this AVP receptor subtype is predominant within the POA of the female rat. In this study, in situ hybridization histochemistry was used to examine V(1a) mRNA expression, either by itself or together with LHRH or glutamic acid decarboxylase 65 (GAD(65)) mRNA, within the POA of ovariectomized rats in the presence or absence of oestrogen. V(1a) mRNA was found in cells across the rostro-caudal axis of the POA; some were in close proximity to cells expressing LHRH mRNA. Coexpression of V(1a) and LHRH mRNAs was detected only very rarely. By contrast, cells with V(1a) mRNA commonly displayed GAD(65) mRNA. The density of V(1a) mRNA-expressing cells was particularly high within the anteroventral periventricular nucleus; at this site, V(1a) mRNA expression was elevated following oestrogen treatment. The present results indicate that V(1a)-mediated AVP actions may influence LHRH release via cells in the immediate vicinity of LHRH neurones and/or via oestrogen-regulated cells in the anteroventral periventricular nucleus, which is a site that lacks LHRH neurones but plays an essential role in initiating the preovulatory LH surge.