Indentifying genetic factors modulating skeletal muscle characteristics can provide an insight into health and disease. A study of inbred strains of mice is a starting point of such an effort. Genetic variations among strains often result in differences in muscle size² and exercise performance³, but the cellular mechanisms behind these differences are not clear. The aim of the study was to investigate activity of citrate synthase (CS), a marker for mitochondrial oxidative capacity⁴, and levels of protein kinase B (PKB) and extracellular signal-regulated kinase 1 and 2 (MAPK(erk1/2)), as two growth regulators¹, in quadriceps muscle of the A/J, BALB/cByJ, DBA/2J, C3H/HeJ, C57BL/6J and PWD/PhJ mice. The quadriceps muscle of 14 week old male mice was dissected after sacrifice and snap frozen. 40-70 mg of the muscle were homogenised and centrifugated. The supernatants were taken and the protein concentration was measured using the Bradford assay. The CS activity was assessed applying a standard spectrophometric assay. The analysis of the specific proteins was performed by the Western blotting. One way ANOVA showed that CS activity (n=9 per strain) was strain dependent (P<0.0001) with A/J mice showing lower values (P<0.01, Tukey’s test) than the other strains. The difference between the A/J and PWD/PhJ mice was particular marked (281±60 versus 731±164 μmol (g protein)^-1min^-1, mean±S.D., respectively). The ANOVA also showed that the Strain factor had a significant effect on the level of PKB (n=5 per strain, ANOVA, P<0.01). Muscles from mice of the PWD/PhJ strain had lower levels of PKB than muscles in the A/J, BALB/cByJ or DBA/2J mice (P<0.01, Tukey’s test). There were no significant differences in MAPK(Erk1/2) between the strains. Results of the study show that the genetic variations influence CS activity and levels of some growth regulators in muscles of mice. This might have a direct effect on muscle morphology and function. Thus studies of inbred mice can improve our understanding of the underlying mechanisms of variation in muscular function.