We have studied mitochondrial adaptations in muscle subject to chronic denervation, and their relationship to muscle performance, using a model of unilateral sciatic nerve denervation in rats over periods of 2, 5, 8, 14, 21, 28, 35, and 42 days (n = 5-9 rats/day). Time to peak tension (TPT), one-half relaxation time (1/2RT), and endurance performance were evaluated during in situ stimulation of denervated and contralateral gastrocnemius-plantaris muscles. Denervation led to a 70% decline in muscle mass after 42 days. TPT and 1/2RT increased 17 and 30%, respectively, indicating a transformation toward slower muscle. The activities of the enzymes cytochrome-c oxidase (CYTOX), succinate dehydrogenase, and citrate synthase were decreased by 8-14 days, and by 42 days these were 34-58% of control. The mitochondrial phospholipid cardiolipin was reduced earlier, by 5 days, and gradually decreased to 37% of control. Thus phospholipid removal appears to precede the loss of enzyme activity during decreases in mitochondrial content. Endurance performance was reduced in parallel with decreases in enzyme activity and cardiolipin. Cytochrome c mRNA levels decreased to 52% of control by 5 days. Denervation resulted in coordinated changes in mRNA levels encoding the nuclear-derived CYTOX subunit VIc and the mitochondrially derived CYTOX subunit III. However, changes in CYTOX activity did not always parallel alterations in subunit mRNA levels. Thus transcriptional and translational mechanisms operate in regulating mitochondrial gene expression during denervation.