Our understanding of the role played by neurotransmitter receptors in the developing brain has advanced in recent years. The major excitatory and inhibitory neurotransmitters in the brain, glutamate and GABA, activate both ionotropic and metabotropic receptors, and are generally associated with neuronal communication in the mature brain. However, before the emergence of their role in neurotransmission in adulthood, they also act to influence earlier developmental events, some of which occur prior to synapse formation. Recent evidences have showed that, in particular, glutamate and GABA metabotropic receptors are strongly expressed during embryonic corticogenesis and that their function may be important for controlling distinct developmental processes. We have demonstrated that GABA_B receptors are involved in the correct migration of GABAergic cortical interneurons (López-Bendito et al., 2003). Moreover, using organotypic slice cultures as well as immunohistochemistry we have observed alterations in the development of thalamocortical axons in mice deficient in GABA_B1 subunit, suggesting that this receptor may also be involved in axon guidance processes. It now appears that there is a differential expression of glutamate and GABA receptor subunits during embryonic, postnatal and adult brain. These changes may be related to the crutial role of these receptors in early developmental events such as proliferation, axon guidance or neuronal migration.