We tried to simulate Ca²⁺ activated Cl^– current in isolated single cardiocytes in pulmonary vein. We could record the transiently activated current by the application three step voltage pulses (holding potential : -40 mV, -80 mV for 50 ms, 30 mV for 5 ms, 10 mV steps from -70 mV to 60 mV) and found this transient current was a Ca²⁺-activated Cl^–-current. For the simulation of this current, all information of the cellular geometry, the intracellular Ca²⁺-regulation and the membrane ionic current systems must be exactly specified. The mean capacitance, length, width and depth are 39.4±3.3 pF (n=30), 116.56±3.7 μm, 10.61±0.34 μm and 6.15±0.24 μm, respectively (mean±S.E.M, n=37), which suggested no existence of t-tubular system and a similar geometry to atrial myocytes (Hüser et al, 1996). Cytosolic application of 1 μM Ca2+ did not activate the Ca²⁺-activated Cl^–-current, which suggested that the cytosolic Ca²⁺ is probably compartmentalized. Na⁺-Ca²⁺ exchange current amplitude was used to calculate the required subsarcolemmal Ca²⁺ concentration. Two releasable sites of the sarcoplasmic reticulum (SR) in atrial myocytes such as junctional SR and central SR were identified (Kockskämper et al., 2001). From those structural data, we composed six compartments for Ca²⁺ regulation such as cytosolic, subsarcolemmal, junctional, junctional SR, central SR and network SR. We incorporated the kinetics of ryanodine sensitive Ca²⁺ release channel based on the report (Fill et al. 2000; Györke & Györke, 1996) with the modification of the kinetics. The L-type Ca²⁺ current kinetics were reconstructed based on our data and the modified version of Shirokov et al. (1993) by Matsuoka et al. (2003). The Na⁺-Ca²⁺ exchange current kinetics and SERCA kinetics were incorporated Matsuoka et al. (2003). We adjusted the amplitude of all related components and successfully simulated Ca²⁺-activated Cl^–-current. From the simulation, we found the junctional [Ca²⁺] can increase up to hundreds μM and the subsarcolemmal [Ca²⁺] can increase up to several tens μM. The Km value of Ca²⁺-activated Cl^–-current for Ca²⁺ may be over 50 μM.