Citrate synthase (CS) is a key enzyme of the Krebs cycle often used as a marker of aerobic oxidation. Changes in the oxidative capacity in skeletal muscle might be critical for exercise performance. The A/J mouse strain has 50-65% lower CS activity than the C57BL/6J (B6) strain (Ratkevicius, Carroll et al. 2010). This decrease is attributed to mutated Cs gene in A/J strain. Congenic strain B6.A-(rs3676616-D10Utsw1)/Kjn (B6.A) carries the A/J allele in the region of chromosome 10 (containing Cs gene) on otherwise B6 genome. Thus, the B6 and B6.A strains offer a model permitting to investigate if function of skeletal muscle is associated with inherited differences in CS activity. The in vitro function of soleus (SOL) and extensor digitorum longus (EDL) muscles was determined in 18-week old males (M) and females (F) from the B6 and B6.A strains (n=8 per strain and sex). Following sacrifice, muscles were excised and mounted on a force transducer (300C, Aurora Sci., Canada) in Tyrode solution at 25 0C. Peak twitch force (Pt), twitch contraction (CT) and half relaxation time (HRT), and maximum isometric force (P0) were recorded. A fatigue test consisting of 180 isometric contractions every second (250 and 500 ms tetani for EDL and SOL, respectively) at 40 Hz was then carried out. Presented values are means ± SD compared with a 2-way ANOVA (strain and sex). Body weight of B6 and B6.A mice did not differ in M (26±1 vs 26±1 g) or F (22±1 vs 23±2 g). There were no statistically significant differences between the B6 and B6.A mice in SOL muscle Pt (M: 33±2 vs 33±3 mN; F: 32±4 vs 31±3 mN) or P0 (M: 176±8 vs 185±16 mN; F: 173±19 vs 170±15 mN). However, CT (M: 69±8 vs 61±5 ms, P<0.05; F: 79±12 vs 65±9 ms, P<0.05) and HRT (M: 128±20 vs 120±12 ms, P>0.05; F: 182±17 vs 149±24 ms, P<0.05) were shorter in the B6.A strain. None of the assessed EDL parameters differed statistically significantly between the two strains: Pt (M: 41±4 vs 41±4 mN; F: 36±5 vs 36±4 mN), CT (M: 26±1 vs 25±1 ms; F: 26±2 vs 25±1 ms), HRT (M: 32±4 vs 30±2 ms; F: 37±6 vs 32±3 ms), P0 (M: 194±18 vs 197±26 mN; F: 189±25 vs 185±18 mN). A decrease in isometric force during fatigue test was nearly identical between the B6 and B6.A mice in SOL muscle (M: 59±5 vs 60±4 %; F: 59±5 vs 59±4 %) albeit slightly different in the EDL (M: 64±3 vs 62±3 %, P>0.05; F: 67±4 vs 62±3 %, P<0.05). Thus, the inherited reduction of CS activity: 1) does not affect muscle force generating capacity, 2) has limited effect on fatigue in isolated muscle, 3) however it is associated with a shorter contraction and relaxation times in slow-twitch SOL muscle. Although these findings should be interpreted cautiously due to possible effects of other genes residing within the congenic region, it appears that a substantial difference in CS activity exerts modest effects on contractility of skeletal muscle.