Changes in cardiac performances were observed under variations of the arterial and/or venous pressures with K-loading or K-depletion in turtle heart. Hearts were perfused with turtle Ringer-Locke s solutions containing various levels of potassium ion concentration. When venous pressure increased from 4 to 12 cm H₂O, cardiac output increased from 6.2±0.68 to 15.7±0.75 ml/min, concomittantly. On the contrary, cardiac output decreased during the elevation of arterial pressure. Stroke work increased more prominently during the arterial pressure elevation than during the elevation of venous pressure. During K-depletion(1~2 mEq/L), cardiac output increased to two times that of normal K-concentration(3~6 mEq/L) group. Heart rate increased also, but less markedly. In K-loaded(7~8 mEq/L) group, both the cardiac output and heart rate decreased but stroke volume rather increased, because heart rate decrement was disproportionate to that of cardiac output. We concluded that in perfused turtle heart, cardiac output variation was more sensitive to K_depletion whereas heart rate to K-loading.