Key points Pharmacological activation of striatal Group I metabotropic glutamate receptors (mGluRs) increases the occurrence of GABAA‐mediated currents in striatal spiny projection neurones (SPNs). Genetically identified striatal interneurones are depolarized by Group I mGluR activation. Group I mGluR activation elevates intracellular calcium in genetically identified striatal interneurones expressing a genetically encoded calcium indicator. Group I mGluR activation results in increased intracellular calcium in SPNs only after priming with calcium influx. Combined electrophysiology and calcium imaging reveals that mGluR activation is not accompanied by depolarization in SPNs. Abstract In mouse striatum, metabotropic glutamate receptor (mGluR) activation leads to several modulatory effects in synaptic transmission. These effects range from dampening of glutamate release from excitatory terminals to depolarization of divergent classes of interneurones. We compared the action of group I mGluR activation on several populations of striatal neurones using a combination of genetic identification, electrophysiology, and Ca2+ imaging techniques. Patch‐clamp recordings from spiny projection neurones (SPNs) and various interneurone populations demonstrated that the group I mGluR agonist (RS)‐3,5‐dihydroxyphenylglycine (DHPG) robustly depolarizes several interneurone classes that form GABAergic synapses onto SPNs. We further utilized the genetic reporter mouse strain Ai38, which expresses the calcium indicator protein GCaMP3 in a Cre‐dependent manner. Breeding Ai38 mice with various neurone selective, promoter‐driven Cre recombinase mice resulted in GCaMP3 expression in defined cell populations in striatum. Consistent with our electrophysiological findings, group I agonist applications increased intracellular levels of calcium ([Ca2+]i) in all interneurone populations tested. We also found that acute DHPG application evoked a transient, rapid increase in [Ca2+]i from only a small percentage of identifiable SPNs. Surprisingly, this fast [Ca2+]i response exhibited a robust enhancement or sensitization, in a calcium‐dependent fashion. Following several procedures to increase [Ca2+]i, the vast majority of SPNs responded with rapid changes in [Ca2+]i to mGluR agonists in a time‐dependent fashion. These findings extend our understanding on group I mGluR influence of striatal output via powerful, local GABAergic connections in addition to [Ca2+]i dynamics that impact on activity or spike‐timing‐dependent forms of synaptic plasticity.