The early postnatal maturation of pyramidal neurons in layers II/III and V of the rat visual cortex has been examined in an attempt to elucidate some determinants of their mature morphology. Three indices have been quantified using Golgi-impregnated pyramidal cells; densities of spines along apical dendrites, numbers of primary based dendrites and volumes of cell bodies. The mean density of spines on the apical dendrites of all pyramidal neurones increases in a stepwise fashion. The first significant increase occurs between days 6 and 9 and the second, between days 12 and 15; these increases may correlate with the arrival of geniculate afferents and with the opening of the eyes, respectively. In younger animals, the distribution of spines along the apical shafts is relatively even, whereas in older animals, spine density increases significantly over the proximal 125 micrometers portion and is relatively constant over the remaining distal portion. By day 21, layer V pyramidal cells have acquired more primary basal dendrites and larger somatic volumes than layer II/III cells. Furthermore, as the cells mature the rate of change in these characteristics are significantly different for neurons in layer II/III and in layer V. For both cell populations, the mean number of primary basal dendrites increases to a maximum before falling to a steady level, but for neurons in layer V, the maximum is higher and attained three days earlier than for layer II/III cells. Moreover, the increase in volume of cell bodies of layer V neurons begins three days before that of layer II/III cells. This three day phase difference in maturation may reflect the cell birth dates, since autoradiographic evidence indicates that layer V pyramidal neurons reach the cortical plate about three days prior to those which occupy layer II/III in the adult visual cortex.