The application of the glutamate analogl-2-amino-4-phosphonobutyric acid (l-AP4) to neurons produces a suppression of synaptic transmission. Althoughl-AP4 is a selective ligand at a subset of metabotropic glutamate receptors (mGluRs), the precise physiological role of thel-AP4-activated mGluRs remains primarily unknown. To provide a better understanding of the function ofl-AP4 receptors, we have generated and studied knockout (KO) mice lacking the mGluR4 subtype of mGluR that displays high affinity forl-AP4. The mGluR4 mutant mice displayed normal spontaneous motor activity and were unimpaired on the bar cross test, indicating that disruption of the mGluR4 gene did not cause gross motor abnormalities, impairments of novelty-induced exploratory behaviors, or alterations in fine motor coordination. However, the mutant mice were deficient on the rotating rod motor-learning test, suggesting that mGluR4 KO mice may have an impaired ability to learn complex motor tasks. Patch-clamp and extracellular field recordings from Purkinje cells in cerebellar slices demonstrated thatl-AP4 had no effect on synaptic responses in the mutant mice, whereas in the wild-type mice 100 μml-AP4 produced a 23% depression of synaptic responses with an EC50of 2.5 μm. An analysis of presynaptic short-term synaptic plasticity at the parallel fiber→Purkinje cell synapse demonstrated that paired-pulse facilitation and post-tetanic potentiation were impaired in the mutant mice. In contrast, long-term depression (LTD) was not impaired. These results indicate that an important function of mGluR4 is to provide a presynaptic mechanism for maintaining synaptic efficacy during repetitive activation. The data also suggest that the presence of mGluR4 at the parallel fiber→Purkinje cell synapse is required for maintaining normal motor function.