Effects of external pH and potassium concentrations on the electrical and mechanical properties were investigated on rabbit papillary muscle. Papillary muscles were perfused in horizontal chamber with Tris Tyrode solutions and action potential along with isometric tension was recorded simultaneously. Potassium concentrations were varied between 1 and 12 mM at low(6.9), normal(7.4) and high (7.9) external pH. The following results were obtained: 1) On rasing the potassium concentration from 1 to 12 mM resting membrane potentials decreased from -88.8±2.8 to -66.4±1.2 mV at normal pH and the amplitude of action potential decreased from 115.1±0.7 to 97.5±2.8 mV. On lowering the potassium concentration, membrane hyperpolarized and at 1 mM potassium concentration resting potentials were -107±2.2 mV. Duration of action potential especially APD₆₀~APD₉₀ increased(APD₉₀: 214±15.8 ms at 1 mM K⁺ to 287±18.1 ms at 12 mM K⁺). 2) During acidosis membranes hyperpolarized by more than 20 mV within 1 min. and then slow recovery was observed during the following 10 min. During alkalosis membranes depolarized about 10 mV, which were maintained until washing with normal Tyrode solutions. 3) On lowering the external pH(7.9-6.5), duration of action potential increased progressively and it was most prominent at pH 6.5 and K⁺ 1mM. 4) Magnitude of developed tension was 0.6±0.14 g/mm² at normal pH and potassium concentration (stimulus frequency : 60/min). Relative isometric tension to normal value increased along the increment of stimulus frequency(44.2±4.2% at 6/min to 271±86.7% at 180/min). Force-frequency relations were altered quantitatively during the perfusion with different external pH solutions. 5) Developed tension did not show marked variation within the range of 2~8 mM potassium concentrations. Positive inotropism was observed at less than 2 mM K⁺ and negative inotropism beyond 12 mM K⁺ concentrations. From the above results we concluded that the effects of potassium ion concentration on electrical and mechanical properties of rabbit papillary muscle are related to the changes in surface negative charge due to acid base disturbances.