Increases in the number of dendritic spines (DS) and modifications in the morphology of spines are observed through the maturation of pyramidal neurons in the somatosensory cortex of the rat. A two-fold increment in the number of spines occurs during days 15 to 30, but a 20% reduction in the overall number of DS is observed between days 30 and 90 to reach the stabilized values in the rat aged 6 months. The representation in a graph of the distribution of spines along the apical dendrite is adjusted to a curve with the maximal score lying at a distance of 200-300 microns from the cellular body. Beyond this region the number of spines decreases following a gentle descending profile. In addition, modifications in the morphology of DS occur through the development. Mushroom-shaped, stubby and short, thin spines in the 30-day-old rat replace spines with long, thin pedicles characteristic of the early developmental stages. Both variability in the density of DS and individual variability with regard to the percentage of the different types of spines on the apical dendrites are observed among neurons in the same animal even in rats 90 days old and in rats aged 6 months.