Abstract The molecular layer of rat cerebral cortex (midcoronal section) was studied by electron microscopy at 7 to 75 postnatal days. The following parameters were analyzed: total surface area and volume of the presynaptic terminals per unit volume of tissue, total surface area and volume of the synaptic vesicles per terminal, number of terminals and number of synaptic vesicles per terminal, surface area and volume of individual terminals, surface area and volume of individual synaptic vesicles, and length of the postsynaptic thickening. Steady increases were evident during development in the total surface area and volume of the presynaptic terminals and the number of terminals per unit volume of tissue. By contrast, there were decreases in the surface areas and volumes of individual terminals, more particularly during the latter part of development. Increases in the total surface area and volume of synaptic vesicles were prominent from 7 to 15 days and from 21 to 75 days with a temporary plateau between 15 and 21 days. Vesicle numbers per terminal increased consistently throughout development, whereas the surface areas of individual vesicles increased initially, decreased between 15 and 21 days, and changed little thereafter. The volumes of individual vesicles did not change significantly with development. The length of the postsynaptic thickening decreased between 10 and 21 days. Other features of presynaptic profiles were also quantified to provide further information about the distribution of membrane within the presynaptic terminal. These results, coupled with the quantitative analysis of terminal and vesicle parameters, help elucidate possible interrelationships between membrane compartments during synaptogenesis.