From the study of movements of Ca⁺⁺ in frog cardiac muscle, Niedergerke (1963) postulated that Ca⁺⁺ necessary for the cardiac contraction is stored in a specific pool. Langer et al (1967) and DeCaro (1967) also found a close relationship between the change of Ca⁺⁺ flux kinetics and the change of contractile force. According to the studies of several investigators, Ca II (Bailey and Dressel 1968) or phase I and II (Langer 1965, Langer et al 1967, 1971) in the Ca⁺⁺ washout curve was associated with cardiac contractility. This investigation was aimed to elucidate the anatomical region of the contractile active Ca⁺⁺ pool. At the same time, it was assumed in this study that Ca⁺⁺ in the sarcoplasmic reticulumn represents one of the major intracellular Ca⁺⁺ pool and cardiac contractility was also dependent on the intracellular Ca⁺⁺ concentration. Consequently, this experiment was performed at different temperatures to activate to activate inhibit the deactivating process of activated Ca⁺⁺ in the intracellular space to see if changes in the contractility decay curve existed at different temperatures. The isolated hearts of rabbits and turtles (Amyda maackii) were attached to the perfusion apparatus according to the method employed by Bailey and Dressel (1968). The isolated hearts were initally perfused with a full Ringer solution containing 2 mg/ml of inulin for 1 hr, and then Ca⁺⁺ and inulin-free Ringer solution was perfused while the isometric tension was recorded and a serial sample of perfusion fluid dripping from the cardiac apex was collected for 10 sec throughout experimental period. The above procedure was performed at 23℃, 30℃ and 38℃ on the rabbit heart and 10 ~ 13℃, 10℃, 25℃, 30℃ and 35℃ on the turtle heart. After determination of Ca⁺⁺ and inulin concentration of the samples, the Ca⁺⁺, inulin washout curve and the contractile tensin decay curve were analysed according to the method of Riggs (1963). The results were summarized as follows; 1. In the rabbit heart, there are 2 inulin compartments, 3 Ca⁺⁺ compartments and sing1e exponential decay of contractile tension. In the turtle heart, there are 1 ~ 2 inulin compartments, 1 ~ 2 Ca⁺⁺ compartments and 1 ~ 2 phases of contractile tension decay. The fact that the inulin space was divided into 3 compartments in the washout curve in these hearts indicates the presence of heterogeneity in cardiac perfusion, i.e., overfused and underperfused area. 2. Ca I a9d Ca II in these hearts were found to have Ca⁺⁺ in the ECF compartments because their half times in the washout curves were far smaller than those of the inulin washout curves in the rabbit heart and similar to those of the inulin washout curves in the turtle heart. Ca III in the rabbit heart may have originated from the intracellular Ca⁺⁺ store. But no Ca III in the turtle heart was found. This may be due to the fact that the iutracellular Ca⁺⁺ pool in the turtle heart was too small to detect using this experimental procedure since sarcoplasmic reticulumn in the turtle heart is poorly developed.