To investigate the cellular interactions within the mammalian visual cortex that are important in ocular dominance column formation, we have examined the role of subplate neurons in this process. LGN axons segregate in layer 4 of the cat's visual cortex between the third and sixth postnatal weeks to give rise to the adult pattern of ocular dominance columns. Subplate neurons are a transient population of neurons that sit in the white matter but have extensive projections into the overlying cortex, particularly layer 4, during neonatal life. Many subplate neurons are present at birth, but most are gone by the end of the period of LGN axon segregation. To examine whether these neurons are required for the segregation of LGN axons, we deleted them by intracortical injections of kainic acid either just after LGN axons had grown into layer 4 (first postnatal week) or later, just before the onset of segregation (third postnatal week). The consequences for the patterning of geniculocortical terminals were evaluated by transneuronal transport of 3H-proline injected into one eye at times when segregation would normally be complete. Following deletion of subplate neurons at either age, LGN axons failed to segregate into ocular dominance columns. Following the late deletions only, geniculocortical axons lost their laminar restriction to layer 4 and projected to layers 2 and 3 as well. Deletion of subplate neurons also resulted in long-term changes in the cytoarchitecture of layer 4. These observations suggest that the interactions that mediate segregation of LGN axons within layer 4 of visual cortex are susceptible to influences from subplate neurons. Although the mechanisms by which subplate neurons exert their effect are not yet clear, these experiments strongly suggest that interactions between LGN axons and layer 4 neurons are not sufficient for column formation, and that subplate neurons most likely play a critical role in interactions leading to ocular segregation.