In mammals the cardiac sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA2) plays a major role during cardiac excitation-contraction (E-C) coupling, transporting Ca²⁺ from the cytosol into the lumen of the sarcoplasmic reticulum (SR) and thus promoting the relaxation of the heart. In fish heart, most of the Ca²⁺ comes from the extracellular medium rather than intracellular compartments, and the activity of the SERCA2 is practically inexistent. However, recently physiological and biochemistry studies on the cardiovascular performance of tunas, showed that the elevated heart rates of these fish may rely on increased use of intracellular SR Ca²⁺ stores. More than that, it was also found that bluefin tuna, the specie that encounter the coldest water (~2^oC), displayed the highest SERCA activity, when compared with the other tuna species studied. More recently, data from satellite tracking of the salmon shark (Lamna ditropis) reveal that these fish have a subarctic-to-subtropical niche, and they spend winter periods in waters as cold as 2^o to 8^oC. Functional assays and protein gels reveal that the expression of the cardiac SERCA2 is enhanced in salmon shark hearts and that is 6 times higher than bluefin tuna and as high as rat ventricles, until the temperature of 25^oC. Now we show that vesicles derived from rainbow trout (Salmo gairdneri) ventricles retain a SERCA pump that is able to transport Ca²⁺ at the expense of ATP hydrolysis either in the presence of Pi or oxalate as Ca²⁺-preciptant agents. The addition of 10μM A₂₃₁₈₇ (Ca²⁺ ionophore) totally prevents the formation of a Ca²⁺gradient. Comparisons with the rat SERCA2, measurements of Ca²⁺ uptake at 20^oC, showed that the trout SERCA2 display a higher activity than rat. However, at 35^oC, the activity of the trout SERCA2 ventricles became twice lower than rat ventricles, suggesting a different SERCA2 temperature dependence between trout and rat. Taken together these data suggest that the high activity of the SERCA2 from trout may be important for retaining cardiac function at cold temperatures. This work is supported by NSF and FAPERJ.