Age-related changes in synaptic density in human visual cortical area 17 (striate cortex) were determined, based on counts in material prepared for electron microscopy with the phosphotungstic acid method. The results were correlated with measurements of the volume of striate cortex from celloidin sections. Two periods were defined, one of rapid synapse production which ends at about postnatal age 8 months, and a subsequent longer period of synapse elimination which extends past age 3 years. Exuberant synaptic connections during early childhood may impart to the immature cerebral cortex plasticity which is lost in the adult.