Sarcalumenin (SAR), a Ca²⁺-binding protein located in the longitudinal sarcoplasmic reticulum (SR), regulates Ca²⁺ reuptake into the SR by interacting with cardiac SR Ca²⁺-ATPase (SERCA2a). We have previously demonstrated that SAR deficiency induced progressive heart failure in response to pressure overload, despite mild cardiac dysfunction in sham-operated SAR knockout (SARKO) mice. Since responses to physiological stresses often differ from those to pathological stresses, we examined the effects of endurance exercise on cardiac function in SARKO mice. Wild-type (WT) and SARKO mice were subjected to endurance treadmill exercise training (about 65% of maximal exercise ability for 60 minutes/day) for 12 weeks. After exercise training, maximal exercise ability was significantly increased by 5% in WT mice (n=6), whereas it was significantly decreased by 37% in SARKO mice (n=5). Cardiac function assessed by echocardiographic examination was significantly decreased in accordance with upregulation of biomarkers of cardiac stress in SARKO mice after training. After training, expression levels of SERCA2a protein were significantly downregulated by 30% in SARKO hearts, whereas they were significantly upregulated by 59% in WT hearts. Consequently, SERCA2 activity was significantly decreased in SARKO hearts after training. Furthermore, the expression levels of other Ca²⁺-handling proteins including phospholamban, ryanodine receptor 2, calsequestrin 2, and sodium-Ca²⁺ exchanger 1 were significantly decreased in SARKO hearts after training. These results indicate that SAR plays a critical role in maintaining cardiac function under physiological stresses such as endurance exercise, by regulating Ca²⁺ transport activity into the SR. SAR may be a primary target for exercise-related adaptation of the Ca²⁺ storage system in the SR to preserve cardiac function.