Abstract: We have found that developmental changes through the adolescent period in the rat cerebral cortex provide parallels to those seen in the human cortex. Like humans, the rat cerebral white matter increases during this time due to increases in the number of axons that become myelinated even while the total number of axons decreases. We have preliminary evidence that estrogen decreases the rate of myelination, which results in a sex difference in adult rats. Another parallel to the human cortex is the nonlinear changes in the size of the cortex. We have found that in some cortical regions, female rats show decreases in cortical volume and number of neurons across the time of puberty, and removal of the ovaries stops these decreases. The rat cortex may serve as a model for the cellular changes underlying the volume changes seen in adolescent humans.