External or internal application of proton on the calcium current was investigated using the whole-cell patch clamp technique in single ventricular cells of the guinea pig. The effect of acid pHo on the calcium current depended on the strength of the buffer in the internal solution. In the presence of 5 mM HEPES in the pipette, external acidification depressed calcium current with the half maximum inhibition of pHo 5.5. When the intracellular pH was buffered strongly to pH 7.2 by 50 mM HEPES within the pipette solution, half maximum inhibition of the calcium current was shifted to the acidic side markedly. When acid was applied in the patch electrode, calcium current was hardly affected by the pH values down to 6.0 in the pipette solution but was depressed by approximately half on further acidification to pH 4.5. When the Na+-H+ exchange system was blocked by superfusing the cell with either amiloride or Na+-deficient solutions (Tris substituted), calcium current was decreased by half at a pipette pH of around 6.5 and was completely and irreversibly blocked at pHpip 6.0. From the above results, we concluded that the calcium current of the ventricular cell is much more sensitive to intracellular protons than extracellular ones.