Lowering extracellular magnesium induces different patterns of epileptiform activity in rat hippocampus and entorhinal cortex. Short recurrent epileptiform discharges in the hippocampus are stable over time, whereas seizure-like events (SLEs) in the entorhinal cortex, the subiculum, and the neighboring neocortex develop into late recurrent discharges which are not blocked by clinically employed antiepileptic drugs. We tested the sensitivity of the different epileptiform discharge patterns to N-methyl-D-aspartate (NMDA)- and non-NMDA-receptor antagonists. As NMDA-receptor antagonist we used dextrorphan, ketamine, and 2-aminophosphonovalerate (2APV); as alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA)-receptor antagonist we employed the quinoxaline derivative glutamate 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). The findings show that the different patterns of epileptiform activity, including the late recurrent discharges, are sensitive to all NMDA-receptor antagonists. However, when dextrorphan was employed to suppress seizure-like events, later recurrent discharges did not develop during the remaining time course of the experiment. CNQX reversibly suppressed recurrent discharges in the hippocampus and SLEs in the entorhinal cortex. However, late recurrent discharges become insensitive to CNQX, even at a high concentration of 60 microns. This finding suggests a prominent role for NMDA receptors in the generation of late recurrent discharges.