The effect of metabotropic glutamate receptor (mGluR) activation on vasopressin (VP) and oxytocin (OT) release was evaluated using explants of the hypothalamoneurohypophysial system. (+/)-1-Aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD), an agonist at groups I and II mGluRs, increased VP and OT release in a concentration-dependent manner. A role for group I mGluRs in VP and OT release was demonstrated by the ability of a group I-specific mGluR antagonist, 1-aminoindan-1,5-idicarboxylic acid (AIDA), to block the effect of t-ACPD and the ability of a group I-specific agonist, (R,S)-3,5-dihydroxyphenylglycine, to significantly increase both VP (P = 0.0029) and OT (P = 0.0032) release. However, AIDA did not alter VP or OT release induced by a ramp increase in osmolality of the perifusion medium. The role of group III mGluRs was examined using L(+)-2-amino-4-phosphonobutyric acid (L-AP4), an agonist of these receptors. L-AP4 did not change basal release of VP or OT and did not prevent osmotically stimulated hormone release. Thus mGluR activation stimulates VP and OT release, but it is not required for osmotic stimulation of hormone release.