A fast synaptic potential mediated by NMDA and non-NMDA receptors. J. Neurophysiol. 78: 2693-2706, 1997. Excitatory synaptic transmission in the CNS often is mediated by two kinetically distinct glutamate receptor subtypes that frequently are colocalized, the N-methyl--aspartate (NMDA) and non-NMDA receptors. Their synaptic currents are typically very slow and very fast, respectively. We examined the pharmacological and physiological properties of chemical excitatory transmission at the mixed electrical and chemical synapses between auditory afferents and the goldfish Mauthner cell, in vivo. Previous physiological data have suggested the involvement of glutamate receptors in this fast excitatory postsynaptic potential (EPSP), the chemical component of which decays with a time constant of <2 ms. We demonstrate here that the pharmacological and voltage-dependent characteristics of the synaptic currents are consistent with glutamatergic transmission and that both NMDA and non-NMDA receptors are involved. The two components surprisingly exhibit quite similar kinetics even at resting potential, with the NMDA response being only slightly slower. Due to its fast kinetics and characteristic voltage dependence, NMDA receptor-mediated transmission at these first-order synapses contributes significantly to paired pulse and frequency-dependent facilitation of successive fast EPSPs during high-frequency repetitive firing, a presynaptic impulse pattern that induces activity-dependent homosynaptic changes in both electrical and chemical transmission. Thus NMDA receptor kinetics in this intact preparation are suited to its functional requirements, namely speed of information transmission and the ability to trigger changes in synaptic efficacy.