We have made patch-clamp recordings from slices of fetal and postnatal rat neocortex in order to study the initial expression and activation of NMDA channels. Recordings from both whole cells and outside-out patches indicated that functional NMDA channels are expressed on neurons within the cortical plate, but not on younger cells within the ventricular zone. The NMDA channels on cortical plate neurons had a unitary conductance of approximately 40 pS, had a mean open time of approximately 6 msec, required glycine to open, and were blocked in a voltage-dependent manner by magnesium. These precocious channels were present before the appearance of functional synaptic activity, yet like NMDA channels in the mature neocortex, they were spontaneously activated by an agonist within brain slices. These results demonstrate that NMDA channels are initially expressed on neocortical neurons some time between the last mitotic division within the ventricular zone and completion of migration into the cortical plate. These early NMDA channels have properties characteristic of NMDA channels on more mature neurons and are similarly activated by an endogenous agonist in situ. Their early appearance and activation indicate that NMDA channels may play a role during early stages of cortical development.