This study describes the measurement of N-methyl-D-aspartate (NMDA)-mediated excitatory postsynaptic potentials (EPSPs) of the CA1 subregion of transverse hippocampal slices from immature and adult rats. Our methods permit extracellular measurement of NMDA-mediated depolarizations in the presence of magnesium (Mg2+) ions. In comparison to slices from adult rats (75-90 days old), NMDA EPSPs in hippocampus from immature rats (25-35 days old) were of significantly greater amplitude and were significantly less sensitive to magnesium. It is suggested that developmental plasticity may be related to changes in magnesium regulation of the NMDA channel complex.