Glutamate AMPARs are major excitatory receptors in the vertebrate CNS. In many biological systems there is a developmental speeding in AMPAR kinetics, which occurs either because of a switch in AMPAR subunits or a change in synaptic morphology. We studied the development of AMPAR-mediated miniature excitatory postsynaptic currents (AMPAR-mEPSCs) in zebrafish Mauthner cells (M-cell) to determine the reasons underlying the speeding of AMPA mEPSCs in this preparation. We recorded AMPAR-mEPSCs in zebrafish ranging in age from 33 hours postfertilization (hpf) to 72 hpf. We found that the glutamate waveform in the synaptic cleft did not change during development suggesting that synaptic morphology played little role in shaping the mEPSC. The current-voltage (I-V) relationship was linear at 33 hpf and outwardly rectified in older animals while AMPAR decay kinetics were slower at positive potentials, compared with negative potentials. The relative change in with depolarization was found to be greater at 48 hpf than at 33 hpf. AMPA receptors in 33 hpf fish had a conductance of ~9 pS, and in older fish ~15 pS. Finally, the desensitization blocker, cyclothiazide, increased by ~4 fold in 48 hpf preparations, but only 1.5 fold in 33 hpf fish. These results are consistent with the hypothesis that the major mechanism underlying the developmental speeding in AMPAR kinetics in zebrafish CNS is a switch in receptor subunits. To our knowledge this is the first study to suggest that AMPARs change subunits during development in fish.