We investigated the contribution of sarcoplasmic reticulum (SR) and Na⁺/Ca²⁺ exchanger in the tension-dependent change in the decay of the Ca²⁺ transients (CaT) in euthyroid (Eu) and hyperthyroid (Hy) myocardium. Hy was induced by thyroxine treatment to enhance the rate of SR Ca²⁺ uptake. With the use of the aequorin method, CaT and tension in twitch contraction were simultaneously measured under various conditions (changing muscle length and Ca²⁺ concentration in solution). In both groups, the decay time of CaT (DT) showed a significant dependence on the developed tension, but the tension dependence of DT in Hy was significantly less than in Eu. In the presence of caffeine (3 mM), the tension dependence of DT in Hy became apparent as in Eu. Inhibition of Na⁺/Ca²⁺ exchanger by replacing Na⁺ with Li⁺ did not affect the dependence in Hy. The normalized extra Ca²⁺, which is the Ca²⁺ concentration change in response to a quick length change, in Hy was similar to that in Eu. pCa-tension relations of skinned trabeculae measured at different lengths (1.9 and 2.3 µm) were nearly identical in both groups. These results indicate that the tension-dependent change in the affinity of troponin C for Ca²⁺ works in both Eu and Hy myocardium and that the tension-dependent change in DT is influenced by the Ca²⁺ uptake rate of SR.