Acidic amino acids, such as glutamate and aspartate, are thought to be excitatory transmitters in the cerebral neocortex and hippocampus1–8. Receptors for these amino acids can be classified into at least three types on the basis of their agonists. Quisqualate-preferring receptors and kainate-preferring receptors are implicated in the mediation of synaptic transmission in many regions including the hippocampus9,10 and visual cortex11, whereas N-methyl-D-aspartate (NMDA)-preferring receptors are thought to be involved in modulating synaptic efficacy, for example in long-term potentiation, a form of synaptic plasticity in the hippocampus12–14. In the visual cortex of the cat and monkey, it is well established that synaptic plasticity, estimated by susceptibility of binocular responsiveness of cortical neurons to monocular visual deprivation, disappears after the 'critical' period of postnatal development15–17. Here we report that during the critical period in young kittens, a selective NMDA-receptor antagonist blocks visual responses of cortical neurons much more effectively than it does in the adult cat. This suggests that NMDA receptors may be involved in establishing synaptic plasticity in the kitten visual cortex.