The effect on limbic kindling of N-methyl-D-aspartate (NMDA) injection into the midline thalamus of rats was investigated. Repeated injection of 25 nmol/0.5 microliters NMDA (experimental group) into the massa intermedia (MI), particularly into the reuniens nucleus, caused tonic and/or clonic generalized convulsion associated with temporal limbic EEG seizure discharge. This did not occur after injection of phosphate-buffered saline (PBS, control group). When the animals were subjected to subsequent kindling at either the hippocampus (HP) or the amygdala (AM), only the experimental group showed a significant facilitation of kindling rate. The results suggest that an NMDA receptor in the reuniens nucleus participates in modulation of temporal limbic excitability and seizure development.