The sarcoplasmic reticulum (SR) is composed of two fractions: the heavy fraction (HSR), which contains proteins involved in Ca²⁺-release like the Ryanodine Channel (RyaC), and the light fraction (LSR), which is enriched in Ca²⁺-ATPase (SERCA), an enzyme responsible for  the Ca²⁺ transport from cytosol to the SR lumen using the energy from ATP hydrolysis. In addition, SERCA 1 is also able to hydrolyze ATP without pumping Ca²⁺, producing more heat per ATP cleaved than that released during the coupled ATPase activity. In this work, we measured the kinetic parameters and the heat production by SERCA 1 from LSR and HSR derived from rabbit white skeletal muscle. The heat generation was quantified using an OMEGA Isothermal Calorimeter at 35^oC.We found that the addition of KCl activates the Ca²⁺-release from HSR and has no effect in LSR. As a result, the amount of Ca²⁺ accumulated at steady state by HSR is lower compared to LSR while the ATPase activity is greater. Although we observed an increased ATPase activity in HSR, the amount of heat released during ATP hydrolysis by this fraction is significantly lower than that measured in LSR. Thus, the calorimetric DH, which reflects the amount of heat per ATP cleaved, is significantly lower in HSR, varying between 10-13 kcal/mol, compared to 18-20 kcal/mol measured in LSR. When the Ca²⁺-efflux is blocked with Ruthenium Red (RR) or high concentrations of MgCl₂ (5mM), conditions that inhibit the RyaC,  the amount of Ca²⁺ accumulated by HSR increases and the ATPase activity decreases. Interestingly, the heat released remains the same as control vesicles. Consequentely, the DH values in the presence of RyaC blockers are significantly increased compared to control (17-25 Kcal/mol compared to 10-12 Kcal/mol). Moreover, if KCl is omitted from the assay medium, there is no Ca²⁺-efflux and the DH is similar to that measured in the presence of Ryanodine Recpetor blockers.  In LSR, the addition of RR or 5mM of MgCl₂ and the absence of KCl have no effect in the DH. Therefore, we conclude that in HSR, the amount of heat released per ATP cleaved depends on whether the Ca²⁺-efflux by Rya Receptor is activated.  This work was supported by CNPq and PRONEX.