Glutamate receptors have been linked to the regulation of several developmental events in the CNS. By using cortical slices of early postnatal mice, we show that in layer I cells, glutamate produces intracellular calcium ([Ca2+]i) elevations mediated by ionotropic and metabotropic glutamate receptors (mGluRs). The contribution of mGluRs to these responses was demonstrated by application of tACPD, an agonist to groups I and II mGluRs, which evoked [Ca2+]i increases that could be reversibly blocked by MCPG, an antagonist to groups I and II mGluRs. In the absence of extracellular Ca2+, repetitive applications of tACPD or quisqualate, an agonist to group I mGluRs, elicited decreasing [Ca2+]i responses that were restored by refilling a thapsigargin-sensitive Ca2+ store. The use of specific group I mGluR agonists CHPG and DHPG indicated that the functional mGluR in layer I was of the mGluR1 subtype. Subtype specific antibodies confirmed the presence of mGlur1α, but not mGluR5, in Cajal-Retzius (Reelin-immunoreactive) neurons.