Synaptic density in human striate cortex was determined at various ages, utilizing a computer assisted method. Simultaneous measurement of total volume of striate cortex made it possible to estimate total number of synapses. Synaptogenesis in human striate cortex was found to be most rapid between ages 2-4 months, a time which also is critical for the development of function in visual cortex of the infant. Synapse elimination occurred subsequently with loss of about 40% of synapses between ages 8 months and 11 years. Synapse numbers were stable in adults, except for a slightly lower value in a single brain at age 71 years. Analysis by individual cortical layers showed similar age related changes in all strata of striate cortex, except for somewhat later synaptogenesis in cortical layers V and VI. Total volume of striate cortex reached adult size remarkably early, at about age 4 months. The findings support the hypothesis that exuberant synaptic connections are an anatomical substrate for plasticity in developing cerebral cortex.