The effect of nerve growth factor (NGF) on the expression of neurofilament and Thy-1 genes in rat PC12 pheochromocytoma cells was examined at both the transcriptional and post-transcriptional levels. Addition of NGF to cultured PC12 cells produced increases in mRNAs corresponding to the 68 kd neurofilament protein (NF68) and the Thy-1 glycoprotein within 24 h, with maximal effects of some 90- and 45-fold stimulation (relative to beta-actin mRNA) being observed after 12 and 4 days of treatment, respectively. In addition, transcriptional run-off analyses using isolated nuclei showed that NGF treatment resulted directly in 8- and 4-fold increases in the rate of NF68 and Thy-1 gene transcription. These gene activation events were independent of overt morphological differentiation of PC12 cells occurring both under conditions permissive and non-permissive for neurite outgrowth, and once established the new molecular phenotype was dependent upon the continued presence of NGF. This is the first molecular evidence for the reversible activation of neuron-specific genes during NGF-induced differentiation in PC12 cells.