A wide variety of conformationally constrained glutamate analogues, active as group I or group II metabotropic glutamate receptor agonists, were employed in a molecular modeling study aimed at the definition of group I and group II agonist pharmacophoric models. The results of this study can be summarized as follows: (i) Recognition sites of both group I and group II mGluRs can adequately be described by five-point pharmacophores. (ii) An extended conformation of glutamate is required for interaction with both group I and group II mGluRs. Group I receptors, however, can also be activated by a more folded conformation if only four pharmacophore points are considered. (iii) Conformational preferences are, however, not sufficient to explain the potency and selectivity of the whole set of ligands. Volume comparison analysis allowed us to define steric environments for group I and group II mGluRs. Group I mGluRs are characterized by a region of allowed volume in proximity of the distal acidic function, whereas group II mGluRs are characterized by a small polar pocket whose occupancy confers high potency and selectivity. Finally, our study points out the necessity of a careful analysis of the energetic requirements needed to attain the putative bioactive conformations and of explicitly considering the conformational mobility of carboxylate groups.