The major excitatory neurotransmitter in the CNS is glutamate. It binds to and activates fast, ligand-gated receptors such as AMPA and NMDA ionotropic receptors. AMPA receptors are composed of 4 subunits (GluA1–4) with varying stoichiometries. The subunit composition of the AMPA receptors imparts different properties to the receptor channels such as time course, conductance states, rectification, and calcium permeability. In some preparations AMPA receptors appear to switch subunits during development, but the reasons for this switch and the mechanisms that underlie it remain largely unknown. To address these issues, we examine synaptic plasticity phenomena related to AMPA receptor subunit composition in embryonic zebrafish. Specifically, we record AMPA receptor activity associated with Mauthner neurons. This is one of the rare vertebrate preparations in which one can study developmental synaptic plasticity in the same cell from organism to organism.