A large amount of genetic information is devoted to brain development. In this study, the cortical development in rats at eight developmental time points (four embryonic [E15, E16, E18, E20] and four postnatal [P0, P7, P14, P21]) was studied using a rat brain 10K cDNA microarray. Significant differential expression was observed in 467 of the 9,805 genes represented on the microarray. Two major Gene Ontology classes—cell differentiation and cell–cell signaling—were found to be important for cortical development. Genes for ribosomal proteins, heterogeneous nuclear ribonucleoproteins, and tubulin proteins were up-regulated in the embryonic stage, coincidently with extensive proliferation of neural precursor cells as the major component of the cerebral cortex. Genes related to neurogenesis, including neurite regeneration, neuron development, and synaptic transmission, were more active in adulthood, when the cerebral cortex reached maturity. The many developmentally modulated genes identified by this approach will facilitate further studies of cortical functions.