N-methyl-D-aspartate (NMDA) type of glutamate receptors play an important role in activity-dependent plasticity in the developing cortex. However, the physiological patterns of cortical activity that activate NMDA receptors in vivo remain largely unknown. We performed full-band recordings from the barrel cortex of neonatal rats in vivo and found that the dominant pattern of the early activity, network driven spindle bursts, are associated with large amplitude NMDA receptor-dependent delta waves. The major sink of delta waves was in the dense cortical plate, which coincided with the sinks of sensory-evoked responses as well as fast spindle-burst oscillations. Pharmacological analysis revealed major contributions from NMDA and alpha-aminopropionate (AMPA) type of glutamate receptors in the generation of delta waves, whereas fast oscillations primarily involved only AMPA receptors. Our results suggest that the 2 component spindle burst is generated by rhythmic, presumably thalamocortical, synaptic input which entrains an AMPA receptor-mediated fast oscillation and who's summation generates an NMDA and AMPA receptor mediated delta wave. The massive summation of thalamocortical activity during the spindle bursts thus provides a long time window for co-incident activation of cortical neurons by the thalamocortical cells which may contribute to the formation of thalamocortical synapses in the barrel cortex during the critical period of developmental plasticity.