In the cat, areas 17 and 18 interconnect soon after birth. To test the hypothesis that the normal development of area 18 depends on interactions with area 17, unilateral lesions of area 17 were created in newborn kittens, and the animals allowed to mature. Horseradish peroxidase was then injected into both lateral geniculate nuclei. The major abnormalities of area 18 in the lesioned hemispheres were a thinning of specifically layers 2 and 3 and abnormally faint geniculocortical labelling of layer 4. Cell densities in layers 2 and 3 of the lesioned hemispheres were similar to or lower than normal. Neonatal destruction of area 17 therefore produced a selective loss of cells in layers 2 and 3 in area 18 (the layers that normally interconnect with area 17), and may have reduced thalamic innervation of layer 4.