Myostatin is a secreted protein which is known to be a negative regulator of skeletal muscle growth. Numerous studies have demonstrated that down-regulating the expression of myostatin results in an increase in muscle mass. This increase in skeletal muscle is accompanied by a marked change in the muscle fibre composition from one reliant on mitochondrial oxidative metabolism to one more focused on glycolytic metabolism. A comparative proteomics study was undertaken to investigate the effect of this altered metabolism has on the mitochondria from the gastrocnemius muscle of myostatin null mice, compared with those from wild type mice. The majority of the proteins identified showed no significant modulation between the two phenotypes, but gives an interesting insight into observations made in previous studies. Several proteins were shown to be modulated; however, only one of these was able to be identified. This protein which had sequence similarity to aldehyde reductase, was up-regulated in myostatin null mitochondria. The significance of this observation remains to be established. Interestingly, this protein has been implicated in detoxification of harmful products of lipid peroxidation.