1. Metabotropic glutamate receptors (mGluRs) are coupled to various second-messenger systems through guanosine 5'-triphosphate-binding proteins. To date, at least seven mGluRs have been cloned, and these mGluR subtypes can be divided into three major groups on the basis of similarities in amino acid sequence, coupling to second-messenger cascades in expression systems, and pharmacological profiles. These groups include group I (mGluR1 and mGluR5), group II (mGluR2 and mGluR3), and group III (mGluR4, mGluR6, and mGluR7). 2. On the basis of its selective activation of phosphoinositide hydrolysis in brain slices and its ability to activate mGluR1a expressed in Xenopus oocytes, others have suggested that 3.5-dihydroxyphenylglycine (DHPG) may be selective for group I mGluRs. Consistent with this hypothesis, we report that DHPG also activates mGluR5 expressed in oocytes, whereas it is inactive at mGluR4 and mGluR7 expressed in baby hamster kidney (BHK) cells. The compound (2S,1'R,2'R,3'R)-2-(2.3-dicarboxycyclopropyl)glycine (DCG-IV) activates both mGluR2 and mGluR3 at submicromolar concentrations, whereas it is inactive at mGluR4 and mGluR1, suggesting that this compound may be selective for group II mGluRs. Consistent with this hypothesis, we find that DCG-IV does not activate mGluR5 expressed in oocytes and does not activate mGluR7 expressed in BHK cells. These findings suggest that DHPG and DCG-IV are highly selective agonists for group I and group II mGluRs, respectively. 3. Previous studies that have examined the physiological roles of mGluRs have generally used agonists that do not differentiate between the various subtypes.(ABSTRACT TRUNCATED AT 250 WORDS)