The observed decrease in respiration during rosette leaf maturation of Nicotiana sylvestris wild-type (WT) plants was shown to be because of a decline in the cytochrome oxidase (COX) pathway activity, measured by 18O/16O oxygen discrimination, while the alternative oxidase pathway (AOX) remained stable. This suggests a higher contribution of the COX pathway to growth respiration than to maintenance respiration. Mitochondrial superoxide dismutase (MnSOD) activity paralleled the decrease in COX activity with leaf age, whereas chloroplastic FeSOD activity increased. Age-dependent respiratory changes were much less apparent in the Cytoplasmic Male Sterile II (CMSII) mitochondrial mutant devoid of respiratory complex I and previously shown to possess increased AOX content and enhanced respiration but lower photosynthesis in mature leaves. Respiration declined less rapidly with leaf age in CMSII than in the WT, and was significantly higher in the mutant when compared with the WTin mature leaves only. In contrast, photosynthesis was lower in the mutant than in the WTat all leaf stages. The higher respiration of mature CMSII leaves was supported exclusively by enhanced COX activity, in association with an increased mitochondrial MnSOD activity. Steady-state levels of AOX1 transcripts increased in maturing WT leaves, and the CMSII mutant had higher amounts of coxI, AOX1 and MnSOD transcripts than the WT. Enhanced activity of the proton-pumping COX route in the mutant can be viewed as a compensation for the lack of the first coupling site of the respiratory chain. However, this is not quite sufficient to ensure normal growth rates in the mutant.