Pharmacologically isolated, NMDA receptor-mediated population EPSPs (pEPSPs) were evoked from area CA1 of hippocampal slices using electrical stimulation of the Schaffer collateral/commissural fiber pathway. Slices were prepared from rats aged 20-25 or 80-100 days. The inhibitory effects of a range of ethanol concentrations were assessed. While ethanol antagonized NMDA-mediated pEPSPs in slices from both age groups, it was significantly more potent against pEPSPs from immature versus mature hippocampi. In slices from mature animals, significant and consistent reduction of pEPSPs was observed only with the highest ethanol concentration (100 mM), whereas 10, 30, or 100 mM significantly reduced the amplitude of pEPSPs in slices from immature animals. These results indicate that NMDA-mediated synaptic activity in the hippocampus is more sensitive to the effects of ethanol in immature versus mature animals.