This study was undertaken to identify the neurotransmitter of the projection from the thalamic mediodorsal nucleus (MD) to the prefrontal cortex (PFC) using both retrograde transport of D-[3H]aspartate and electrophysiological approaches in the rat. Unilateral microinjections of D-[3H]aspartate performed into the prelimbic area of the PFC resulted in dense labelling of numerous cells in the ipsilateral MD. Excitatory responses were observed in PFC neurons after electrical stimulation of the MD. However, since cortical neurons project to the MD, these excitatory responses could have resulted either from the activation of the MD-PFC pathway and/or from the activation of recurrent collaterals of antidromically driven cortico-thalamic fibres. The conduction time of each of these two reciprocal pathways was determined by antidromic activation. Short latency excitatory responses resulted from activation of the MD-PFC pathway. They were predominantly observed in PFC neurons located in layer III and evoked at low frequency stimulation (0.3-1 Hz). These excitatory responses disappeared or were replaced by longer latency responses when higher frequency stimulations (3-10 Hz) were used. MD-evoked responses were blocked by the iontophoretic application of the AMPA receptor antagonist CNQX into the PFC. These results indicate that the MD-PFC pathway utilizes glutamate and/or aspartate as the neurotransmitter and that its activation induces excitation in PFC neurons through AMPA receptors. Even though the local application of the NMDA receptor antagonist APV was ineffective, a contribution of these receptors in MD-PFC transmission cannot be excluded.