Brain derived neurotrophic factor (BDNF) is widely distributed in the central nervous system (CNS) and has survival-promoting actions on a variety of CNS neurons. We have examined changes in the level of BDNF mRNA expression in the dorsolateral prefrontal cortex (DLPFC) of the postnatal human brain using both RNAse protection assay and in situ hybridization. Expression of BDNF mRNA in the DLPFC was compared to that in the occipital cortex. BDNF mRNA levels vary between layers, with layer VI consistently higher than other layers in both the DLPFC and occipital regions. BDNF mRNA levels increase approximately one-third from infancy to adulthood, i.e. they are relatively low during infancy and adolescence, peak during young adulthood, and are maintained at a constant level throughout adulthood and aging. The significant increase in BDNF mRNA levels in the DLPFC during the young adult period coincides with the time when the frontal cortex matures both structurally and functionally. The increase in BDNF at this critical time in human development may have important implications for the etiology and treatment of the severe mental disorders that tend to present during this time.