In mammalian development, the refinement of topographical projections from the thalamus to the cortex is thought to arise through an activity-dependent process in which thalamic axons compete for cortical targets. In support of this view, if activity is altered during a critical period in early development, normal connectivity is disrupted. It has been proposed that synaptic connections are strengthened during development by correlated pre- and postsynaptic activity, and a likely mechanism for this process would be N-methyl-D-aspartate (NMDA) receptor-dependent long-term potentiation (LTP). However, the evidence that LTP is involved in normal development remains inconclusive. We have examined LTP in the thalamocortical synapses that form whisker barrels in rat somatosensory cortex (S1). We report here that the period during which LTP can be induced matches closely the critical period during which the barrels can be modified by sensory perturbations. Moreover, the loss of susceptibility to LTP with age is accompanied by a decrease in NMDA receptor-mediated synaptic currents. These findings provide compelling evidence that LTP is important for the development of cortical circuitry.