Glutamic acid is known to be a major excitatory transmitter in the central nervous system. The nature of its receptors on the spinal motoneuron have been well investigated and elucidated to consist of ionotropic (iGluR) as well as metabotropic glutamate receptors (mGluR). However, the role of mGluR on trigeminal motoneurons (TMN) and trigeminal sensory neurons (TSN), which play important roles in mastication, has not been well investigated. In this study, the nature of mGluR on TMN and TSN was investigated using isolated brainstem preparations from neonatal rats. The activities of TMN and TSN were recorded from the trigeminal motor and sensory roots respectively, using a suction electrode. All iGluR agonists depolarized TMN and TSN. The mGluR agonists used were RS-3,5dihydroxyphenylglycine (RS-DHPG) for group I, (2S, 1'R, 2'R, 3'R)-2-(2, 3-Dicarboxycyclopropyl) glycine (DCG-IV) for group II, and L(+)-2-amino-4-phosphono-propinate (L-AP4) for group III. Whereas group I agonist increased spontaneous firing, groups II and III agonists decreased it. Group I agonist depolarized TMN and TSN directly. In contrast, group II and III agonists hyperpolarized TSN but induced no potencial changes in TMN. These results suggested that mGluR exist on TMN and TSN.