The effects of hypercapnia, together with low and high levels of extracellular Ca(2+), on heart activity and intracellular pH were examined in isolated perfused hearts from the land snail Helix lucorum. In addition, the intracellular level of Ca(2+) was determined in slices of ventricles superfused with both normal and hypercapnic salines, containing low and high concentrations of Ca(2+), to investigate whether low extracellular pH affects the entry of Ca(2+) into the heart cells. We also examined the effect of a saline that simulated the composition of the haemolymph of snails after estivating for 3 months on the heart activity and intracellular pH. The results showed that hypercapnia causes decreases in the rate and force of heart contraction, and these are more pronounced in the presence of low levels of extracellular Ca(2+). Moreover, the present results indicate that Ca(2+) maintains the contractility of the heart muscle under acidic conditions and seems to act by competing with protons for the Ca(2+ )binding sites on sarcolemma. The negative effect of hypercapnia on heart activity appears to be due to a reduction in extracellular pH rather than to changes in intracellular pH.