There is evidence that the effect of extracellular Ca²⁺ on heart rate is temperature-dependent: at 38℃ excess Ca²⁺ induces positive chronotropic response, whereas at 30℃ there is no significant chronotropic effect of Ca²⁺. The cause of this temperature-dependency, however, remains still unclear. Therefore, this study was undertaken to investigate the chronotropic effect of external Ca²⁺ at different temperature in the isolated rabbit atria and in the small strips of SA node cut perpendicularly to crista terminalis. In the isolated atria, the Ca²⁺ effect was temperature-dependent: at 35℃ excess Ca²⁺ evoked positive chronotropic response, while at 30℃ there was no significant changes in sinus rate. On the contrary, in the small SA strips external Ca²⁺ induced negative chronotropic effect. At 35℃ changes in Ca²⁺ concentration from 2 to 4, 6, and 10 mM decreased the sinus rate by 2.7±1.6%, 11.2±3.7% and 23.2±8.1% respectively. Lowering the temperature to 30℃, the negative chronotropic effect of Ca²⁺ became greater. With intracellular microelectrodes transmembrane potential was recorded in the small SA strips at 30℃, 35℃ and 38℃. As temperature increased from 30 to 38℃, sinus rate was accelerated by 13/min/℃, APD₅₀(action ptential duration from peak to 50% repolarization) decreased by 5 msec/℃, and amplitude of action potential was slightly decreased. With an increase in Ca²⁺ concentrations from 0.5 to 6 mM, overshoot increased and MDP decreased. These Ca²⁺ effects on the overshoot and MDP of action potentials were not altered by temperature. But the Ca²⁺ effects on the rates of diastolic depolarization, systolic depolarization and repolarization were modified by temperature. Discrpancy of the chronotropic effects of Ca²⁺ between isolated atria and small SA strips was discussed.