It is becoming increasingly evident that significant changes in gene expression occur during the course of neuronal differentiation. Thus, it should be possible to gain information about the biochemical events by identifying differentially expressed genes in neuronal differentiation The PC12 cell line is a useful model system to investigate the molecular mechanism underlying neuronal differentiation and has been used extensively for the study of the molecular events that underlie the biological actions of nerve growth factor (NGF). In this study, we report an application of the recently described mRNA differential display method to analyze differential gene expression during neuronal differentiation. Using this technique, we have identified several cDNA tags expressed differentially during neuronal differentiation. Interestingly, one of these clones was cytochrome c oxidase subunit I (COX I) gene. The differential expression of COX I gene was confirmed by Northern blot analysis as well as RT-PCR. Southern blot analysis of the genomic DNA of PC12 cells revealed that COX I is a single gene. Induction of the oxidative enzyme might reflect the energy requirement in neuronal differentiation.