Cytochrome oxidase (CO) plays a key role in oxidative capacity of neurons and serves as a sensitive indicator of neuronal activity. The mechanism(s) involved in the regulation of this bigenomic-encoded mitochondrial enzyme is still not clearly understood. Previous studies have shown changes in the level of its subunit mRNAs encoded by the nuclear or mitochondrial genome in response to changing neuronal activity. Our goals in the present study were to determine whether such changes were due to RNA synthesis rate or stability or both. The level of CO activity of neurons in primary cultures assayed histochemically was increased after depolarizing KCl treatment. The steady-state levels of CO subunit II (CO II; mitochondrial-encoded) and IV (CO IV; nuclearencoded) mRNAs were up-regulated in response to 5 hr of 20 mM KCl treatment. By using gene-specific probes, the relative rates of synthesis of CO II and IV mRNA were elevated significantly after KCl treatment (P Ο½ 0.05). The degradation of CO II and IV mRNAs was monitored by 3 H-uridine pulse-chase labeling, which revealed halflives of 84 min for CO II mRNA and 50 min for CO IV mRNA. Under KCl treatment, the half-life of CO IV was increased to 102 min, but there was no statistically significant change in the half-life of CO II mRNA. These results indicate that mitochondrial subunit CO II mRNA is regulated mainly at the transcriptional level, whereas the nuclear subunit CO IV mRNA is regulated at both the synthetic and the degradative levels. Both subunits, however, are tightly governed by neuronal activity. J.