The expression and Triton X-100 (Triton) solubility of neuronal intermediate filament proteins were determined in the developing rat cerebral cortex. The level of expression of alpha-internexin was unchanged from embryonic day 15 (E15) to postnatal day 15 (P15), whereas expression of the mid-sized neurofilament subunit increased continually during this interval concomitant with a reduction in Triton solubility of the two proteins. The low molecular weight neurofilament subunit, first barely detected at P2, was largely insoluble in Triton from the initial time point that its solubility could be assayed, at P5, to P15. Similar expression patterns and Triton solubility profiles were obtained for neuronal intermediate filament proteins in cultured neurons from E15 cerebral cortex. These results suggest that alpha-internexin is expressed earlier than neurofilament proteins to provide a more plastic network in the early developing brain. The incorporation of neurofilament proteins apparently results in the formation of the more stable intermediate filament network found in mature neurons.