The role of external Ca2+ in the homeostasis of intracellular pH (pHi) of Anabaena sp. strain PCC7120 in response to a decrease in the external pH (pHex) has been studied in cell suspensions. Increase in cytoplasmic pH after acid shock is dependent on the presence of Ca2+ in the medium. The observed Ca2+-mediated alkalization of the cytoplasm depends on the extent of the shift in external pH. Acid pH shifts resulted in an increased permeability of the cytoplasmic membrane to protons, which could be reversed by increasing the concentration of Ca2+ in the medium. Thus, the ability of Ca2+ to increase cytoplasmic pH might be correlated with an inhibition of net proton uptake by increasing concentrations of external Ca2+ under these conditions. This combined response resulted in the generation and maintenance of a larger pH gradient (ΔpH) at acid external pH values. All Ca2+ channel blockers tested, such as verapamil and LaCl3, inhibited the observed Ca2+-mediated response. On the other hand, the Ca ionophore calcimycin (compound A23187) was agonistic, and stimulated both cytoplasmic alkalization and inhibition of net proton uptake. The protonophorous uncoupler carbonylcyanide m-chlorophenyl hydrazone, inhibited this Ca2+-mediated response, whereas monensin, an inhibitor of the Na+/H+ antiporter, had no significant effect. The results of the present study suggest that an influx of Ca2+ from the extracellular space is required for the regulation of cytoplasmic pH in Anabaena sp. strain PCC7120 exposed to low external pH values.