A superfusion system was used in order to investigate the ionic requirements of luteinizing hormone releasing hormone (LHRH) and somatostatin (SRIF) release from mediobasal hypothalamic (MBH) slices of male adult rats. Slices were superfused with Hepes-buffered Locke medium at 37 °C in an atmosphere of Ch 95%–CO2 5% for 1 h. Bacitracin (2 × 10<sup>–5</sup> M) was added to the medium to prevent enzymatic degradation of neuropeptides. Depolarizing agents such as potassium (K<sup>+</sup>) or veratridine stimulated LHRH and SRIF release in a dose-dependent manner. Maximal effect was obtained with K<sup>+</sup> 56 m M and veratridine 50 µ M. The depolarizing effect of K<sup>+</sup> 56 m M was specific and not due to the hypertonicity of the medium used. Neither Mg<sup>2+</sup> nor chlorine was needed for the spontaneous or K<sup>+</sup>-evoked release of LHRH and SRIF. The amplitude of the secretory response to K<sup>+</sup> 56 m M was related to Ca<sup>2+</sup> concentration tested in the range of 0.2–8.8 m M; maximal responses were obtained between 0.8 and 1.8 m M. Removal of Ca<sup>2+</sup> from the medium with or without replacement by Mg<sup>2+</sup>, as well as administration of voltage-sensitive Ca<sup>2+</sup> channel blockers (D-600 10<sup>–4</sup> M, Mn<sup>2+</sup> 3 mM) blocked both K<sup>+</sup> and veratridine induced neuropeptide release. When sodium channels and the ‘early’ calcium channels were blocked by tetrodotoxin (5 × 10<sup>–7</sup> M) the stimulatory effect of veratridine was completely blocked whereas the stimulation of K<sup>+</sup> was unaffected. These experiments indicate that: (1) both K<sup>+</sup> and veratridine induced-LHRH and SRIF release is a Ca2<sup>+</sup>-dependent process; (2) Ca<sup>2+</sup> concentration is critical for the amplitude of the secretory response; (3) the main Ca<sup>2+</sup> channel involved in neuropeptide release corresponds to the voltage-sensitive Ca<sup>2+</sup> channel, and (4) neither magnesium or chlorine is needed for either spontaneous or evoked release of LHRH and SRIF.