Histamine depolarises vestibular neurones via H1 and H2 receptors (Wang & Dutia, 1995), and infusion of H2-receptor antagonists or H3-receptor agonists into the medial vestibular nucleus (MVN) induces a behavioural syndrome similar to that seen after unilateral labyrinthectomy (UL; Yabe et al. 1993). Presynaptic H3-receptors can act both as inhibitory autoreceptors and as inhibitory heteroreceptors, but it is not known if the latter mechanism has a role in vestibular function. To clarify this we investigated the histaminergic regulation of endogenous GABA release in superfused MVN slices. Slices were prepared from normal male Lister hooded rats and from animals which underwent left labyrinthectomy under halothane anaesthesia either 25 h or 7 days earlier (Yamanaka et al. 2000). The animals were decapitated under halothane anaesthesia and horizontal 400-450 ¼m slices of the MVNs of the two sides were prepared in chilled artificial cerebrospinal fluid (aCSF; 123 mM NaCl, 5 mM KCl, 2.4 mM CaCl₂, 1.3 mM MgSO₄, 26 mM NaHCO₃, 1.2 mM KH₂PO₄, 10 mM D-glucose, 100 ¼M glutamine). The slices were separately superfused with oxygenated aCSF at 32°C, and GABA release was determined with high performance liquid chromatography. In samples from normal MVN slices, the GABA release evoked by a 4-min stimulus with high K⁺ (60 mM) aCSF was Ca²⁺ dependent, indicating neuronal origin. Histamine (100 ¼M) reduced the evoked GABA release from 36±5 fmol/mg to 18±2 fmol/mg (mean±SEM, unpaired t test, P=0.0099, df=18). The H3-agonist immepip dose-dependently inhibited evoked GABA release, with a calculated IC₅₀ of 0.34 nM, reaching maximum inhibition (-45% to -70%, 95% confidence interval) at 10100 nM. In superfusates from MVNs prepared from UL animals, an imbalance in evoked GABA release (with less release from the contra-lesional MVN) was observed after 25 h (paired t test, P=0.0093, df=6) but not after 7 days. The inhibitory effect of immepip was attenuated in MVN slices from UL animals at both time points. In slices from 25 h post-UL animals, immepip (100 nM) removed the imbalance of evoked GABA release between the ipsi-lesional and contra-lesional MVNs. This is the first report of H3-mediated inhibition of GABA release in the MVN. The change in immepip potency after UL is in line with changes in H3-receptor mRNA expression after UL (Lozada et al. 2004). Changes in H3-receptor control of GABA release in the MVN may be important in the rebalancing of bilateral MVN activity after unilateral deafferentation.