Objective: Calsequestrin (CSQ) is the main Ca²⁺ binding protein inside the sarcoplasmic reticulum (SR) of skeletal and cardiac muscle. The present study demonstrates the specific effects of different training regimens on CSQ isoform 1 (CSQ1, the primary isoform) expression in various skeletal muscles of mouse. Methods: CSQ1 protein expression was determined with Western blot in m. soleus (SOL), m. extensor digitorum longus (EDL), m. rectus femoris (RF), m. gastrocnemius (GAS), m. tibialis anterior (TA), and diaphragm muscles after completing a six-week sprint or endurance training program. Results: Significant decreases in CSQ1 were observed in SOL, RF, and diaphragm muscles after endurance training (p<0,001), whereas after intensive sprint training the trend was quite the opposite i.e. significantly increasing in GAS (p<0.01). Conclusions: These results indicate that the diverse training strategies used (endurance versus sprint training) affect differently CSQ1 concentrations in the skeletal muscle of mouse. It has previously been observed that Ca²⁺ metabolism is more efficient in fast fibre types (containing CSQ1 isoform), and that the endurance and sprint training divergently alter the fibre type composition of the muscle studied. It is, thus, concluded that in order to maintain a large total pool of releasable Ca²⁺ in the SR, the expression of CSQ1 is up-regulated induced by sprint training, but not endurance exercise. Furthermore, the regulation is most likely associated with the fibre-type changes promoted by the increased activity of muscles.