Synapses have been counted by electron microscopy and neurones by light microscopy through the depth of the visual cortex in a series of cats from 37 days gestation to adulthood. A few definite synapses are present as early as three weeks before birth, but there is then a latent period of four weeks before synapses increase rapidly in number 8-37 days after birth. The synapses occur just above and just below the cell plate at first, but in the adult cat they become evenly distributed in the depth of the cortex. The gradual separation of neurones by neuropil during development precedes a parallel increase in the density of synapses by about one week. The average number of synapses associated with one neurone rises to a peak of about 13,000 at seven weeks after birth. The densities of synapses and of neurones subsequently fall to slightly lower values in adult cats as the glial cells continue to develop. The timing of synaptic development in the visual cortex has been compared quantitatively with that in the L. G. N. and qualitatively with synaptogenesis in the retina. Synapses develop in the L. G. N. and cortex in a parallel fashion, and the L. G. N. precedes the cortex by a short interval of about two days. In the cell plate of the retina a few receptor synapses are present nine days before birth. Inner plexiform synapses are aslo present at this time, but ribbon-containing synapses do not appear until birth. Very few receptors possess outer segments with discs at birth, but five days later disc-bearing outer segments have developed. Thus synaptic development starts before afferent impulses can enter the visual system, but the main increase in synapses in the L. G. N. and cortex takes place four weeks after the start of synapse formation while the visual system is being used.