In the rodent primary somatosensory cortex, the configuration of whiskers and sinus hairs on the snout and of receptor-dense zones on the paws is topographically represented as discrete modules of layer IV granule cells (barrels) and thalamocortical afferent terminals 1,2 . The role of neural activity, particularly activity mediated by NMDARs (N-methyl-D-aspartate receptors), in patterning of the somatosensory cortex has been a subject of debate [3][4][5][6] . We have generated mice in which deletion of the NMDAR1 (NR1) gene is restricted to excitatory cortical neurons, and here we show that sensory periphery-related patterns develop normally in the brainstem and thalamic somatosensory relay stations of these mice. In the somatosensory cortex, thalamocortical afferents corresponding to large whiskers form patterns and display critical period plasticity, but their patterning is not as distinct as that seen in the cortex of normal mice. Other thalamocortical patterns corresponding to sinus hairs and digits are mostly absent. The cellular aggregates known as barrels and barrel boundaries do not develop even at sites where thalamocortical afferents cluster. Our findings indicate that cortical NMDARs are essential for the aggregation of layer IV cells into barrels and for development of the full complement of thalamocortical patterns.The rodent somatosensory system is an excellent model to study molecular mechanisms underlying the establishment of patterned topographic connections between the sensory periphery and the brain. Trigeminal and dorsal column pathways convey a somatosensory map from the periphery to the neocortex by relay stations in the brainstem and the ventrobasal thalamus. At each level, pre-synaptic afferents and their target cells establish a patterned array of modules corresponding to the arrangement of whiskers and sinus hairs on the snout (trigeminal pathway), and receptor-dense zones of the paws (dorsal column pathway) 2,7,8 . These patterns are consolidated during the first postnatal week, and are subject to alterations if the sensory periphery is perturbed by nerve or whisker lesions during a critical period 2,7,8 . Thus, early neural activity might be essential in the development and plasticity of central patterns much like that of the vertebrate visual system 9,10 . Within this context, NMDAR activity has attracted much attention 9,11 . Initial studies using pharmacological blockade of NMDARs in postnatal rat somatosensory cortex did not detect effects on thalamocortical patterning, but revealed functional impairment of thalamocortical connectivity and whisker-specific responsiveness of barrel neurons 4,5 . In contrast, genetic manipulations of NMDARs showed an absence of periphery-related patterns in the somatosensory cortex 6 . This result could not be ascribed to direct effects on the neocortex, however, as it could have been a consequence of impaired patterns at the brainstem level 6,12,13 . To delineate the specific role(s) of cortical NMDARs in patterning, here we disrupted the ...