We have studied the expression of c-myc and c-fos proto-oncogenes in various areas of the central nervous system during postnatal development. c-myc mRNA levels increased during the first 5 days and then decreased over the next 15 days in all nervous regions studied. c-fos mRNA levels changed in a different way in four brain areas. While in cerebral cortex and cerebellum there was a sharp decrease during the first 10 days, in white matter and hypothalamus c-fos transcript levels remained high during the same period, decreasing at a later stage. Changes in oncogenes mRNA levels are related to various developmental events, such as neurite growth, myelination and cell proliferation. The dissimilar patterns of c-myc and c-fos expression suggests that they play different functions in CNS maturation. c-myc mRNA levels are temporally related to active neurite growth and to cell proliferation. Changes in c-fos mRNA correlate in time with early developmental processes and also with those occurring at later stages, such as myelination.