Myostatin is a natural inhibitor of muscle growth (2). Myostatin inhibition could be effective in counteracting muscle degenerative diseases (1) and is being considered for treatment against obesity since animals with dysfunctional myostatin show a decrease in body fat (2, 4). Improved mitochondrial performance plays an important role in resistance to obesity (3). The aim of this study was to investigate if disruption of myostatin function is associated with increased activity of mitochondrial enzymes in skeletal muscles. We compared activity of mitochondrial citrate synthase (CS) and β hydroxyacyl CoA dehydrogenase (HADH) in the gastrocnemius muscles (n=6) of male mice (age ~70 days) from two high growth strains, DUI and BEH, after introgressing the wildtype allele and the Compact (C) allele by marker assisted selection (2, 5). The C allele is associated with loss myostatin function (2, 5). The CS and HADH activity was measured using standard spectrophotometric assays and protein concentration of muscle homogenates was assessed using Bradford assay. The comparisons were done using 2 way ANOVA followed by Benferonni post tests. The data are presented as means ± S.D. There was no significant difference in CS activity between the strains (P = 0.059). The C allele had a negative effect on CS activity (P < 0.01) in both BEH strain (751.7 ± 178.9, 770.9 ± 159.9 and 555.9 ± 101.0 μmol (g protein)-1 min-1 for BEH+/+, BEHC/+ and BEHC/C, respectively) and DUI strain (712.9 ± 210.9, 592.1 ± 118.1 and 478.4 ± 96.1 μmol (g protein)-1 min-1 for DUI+/+, DUIC/+ and DUIC/C, respectively). On the other hand, HADH activity was influenced by both C allele factor (P < 0.01) and Strain factor (P < 0.01). The C allele had a negative effect on HADH activity (P < 0.01) for both BEH strain (9.0 ± 2.3, 10.8 ± 5.0 and 6.1 ± 1.1 μmol (g protein)-1 min-1 for BEH+/+, BEHC/+ and BEHC/C, respectively) and DUI strain (7.1 ± 2.5, 5.7 ± 0.9 and 4.6 ± 0.6 μmol (g protein)-1 min-1 for DUI+/+, DUIC/+ and DUIC/C, respectively). DUIC/+ mice showed lower values of HADH activity than BEHC/+ mice (P < 0.01). In summary, myostatin dysfunction had a negative effect on mitochondrial activity in both BEH and DUI strains. It is likely that myostatin inhibition leads to a decrease in body fat through other mechanism than increased capacity for fat oxidation by skeletal muscles.