Persistent effects of chronic neonatal administration of the N-methyl-D-aspartate (NMDA) antagonist MK-801 were investigated by measuring susceptibility to CA1 kindling epileptogenesis in adulthood. Rat pups were chronically treated with MK-801 from postnatal day 8 through day 19. Hippocampal kindling showed an increase in electrical seizure duration in the MK-801-treated group as compared with controls along with a more severe expression of behavioral seizures during the first few kindling stimulations. These results show that neonatal interference with NMDA receptor function leads to a long-lasting increase in hippocampal excitability.