This project traced the maturation of the human auditory cortex from midgestation to young adulthood, using immunostaining of axonal neurofilaments to determine the time of onset of rapid conduction. The study identified 3 developmental periods, each characterized by maturation of a different axonal system. During the perinatal period (3rd trimester to 4th postnatal month), neurofilament expression occurs only in axons of the marginal layer. These axons drive the structural and functional development of cells in the deeper cortical layers, but do not relay external stimuli. In early childhood (6 months to 5 years), maturing thalamocortical afferents to the deeper cortical layers are the first source of input to the auditory cortex from lower levels of the auditory system. During later childhood (5 to 12 years), maturation of commissural and association axons in the superficial cortical layers allows communication between different subdivisions of the auditory cortex, thus forming a basis for more complex cortical processing of auditory stimuli.