Changes in the subunit composition of the NMDA receptor have been suggested as a mechanism of developmental plasticity in the cortex. We have previously shown in the mouse barrel cortex (Mierau et al., 2001, SfN abstract) that the decay time constant of the NMDA receptor-mediated component of the EPSC in layer 2/3 pyramidal neurons evoked by extracellular stimulation in layer 4 decreased during the second postnatal week. This change is likely to be due to a decrease in the expression of the NR2B subunit in the synaptic NMDA receptors because application of the selective NR2B antagonist ifenprodil (3 μM) decreased the ratio of NMDA to AMPA receptor-mediated current and the decay time constant of the NMDA current in slices from postnatal day (P)6-8 but had no effect on currents at P13-15. To investigate whether this developmental change in synaptic currents depends upon sensory experience, we recorded EPSCs in barrel cortex slices from P13-15 mice with whiskers plucked at P6. In contrast to normal P13-15 mic...