Indirect evidence suggests that excitatory amino acids (EAA) are involved in synaptic transmission of visceral afferents at their synapses within the nucleus tractus solitarius (NTS). Little is known about the identity of the postsynaptic receptors or response mechanisms. Here we report results from a longitudinal brain slice of the rat medulla. Intracellular recordings were made from neurons in delimited portions of the dorsal medial NTS (mNTS) known to receive baroreceptor inputs. Stimulation of the solitary tract 1-3 mm from the mNTS recording site evoked short (2 ms) latency excitatory postsynaptic potentials (EPSPs), which had durations of 40-50 ms. Addition of the non-N-methyl-D-aspartate (non-NMDA) selective antagonist 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX) to the slice surface near the recording electrode resulted in a rapid (within 30-45 s) suppression of the EPSP. Complete EPSP blockade was only slowly reversed by drug-free saline. Concentration-response relations (n = 14) showed 50% depression of EPSPs by surface concentrations of 1-10 microM CNQX. EPSP amplitude was resistant to the selective NMDA antagonist 2-amino-5-phosphonovalerate (AP 5) and, on average, was reduced less than 20% at 100 microM AP 5, an effect that was not statistically significant (n = 10; P greater than 0.05). In conclusion, this study offers the first direct evidence that EAAs mediate the primary events of afferent synaptic transmission in NTS. The experiments suggest that excitatory sensory afferent synaptic transmission to mNTS neurons is mediated by an EAA transmitter acting at non-NMDA receptors, but NMDA receptors may have a modulatory role.