gamma-Aminobutyric acid (GABA) neurones are present in the zona incerta (ZI) where other systems have been shown to influence gonadotrophin release. This report investigates the effect of GABA agents in the ZI on ovulation and luteinizing hormone (LH) release. In intact females under Saffan anaesthesia, bilateral stereotactic injections into the ZI of two GABA transaminase inhibitors [amino(oxy)acetic acid and gamma-acetylene GABA] on the morning of pro-estrus or two GABA agonists on the afternoon of pro-estrus inhibited ovulation. The selective GABA B agonist baclofen was effective at 0.05 nM; muscimol, a mixed GABA A and B agonist, was 50-fold less potent, while the selective GABA A agonist isoguvacine had no effect at 500 nM. Administration of baclofen at 0.05 and 5 nM into the ZI significantly reduced plasma LH concentration in untreated ovariectomized rats and also prevented the rise in LH normally induced in ovariectomised rats primed with 5 micrograms oestradiol benzoate (OB) plus 0.5 mg progesterone. In ovariectomised rats primed with 5 micrograms OB alone, administration of the selective GABA A antagonist bicuculline (200 and 260 pg/side) had no effect on plasma LH, while the GABA B antagonist phaclofen (10 pg/side) stimulated a rise in plasma LH, 40 and 60 min after injection. These results indicate that GABA activity in the ZI exerts an inhibitory effect on LH release and ovulation and this is preferentially exerted via GABA B receptors.