The aims of this work were to determine: 1) whether Ca²⁺ exit via the plasmalemmal Ca²⁺ ATPase (PMCA) is coupled to H⁺ entry via a Ca²⁺/H⁺ exchange; 2) whether operation of PMCA has an absolute requirement on external H⁺ (H_o); and 3) the stoichiometry and voltage-dependence of the Ca²⁺/H⁺ exchange. Barnacle muscle cells were used because of the ease with which they can be internally-perfused (e.g., with ⁴⁵Ca), voltage-clamped and impaled with a pH electrode. Thus, the simultaneous measurement of plasmalemmal Ca²⁺ and H⁺ fluxes can be measured. The effects of H_o, intracellular ATP, PMCA blockers, and membrane potential (V_M) were studied on PMCA-mediated Ca²⁺/H⁺ exchange. The results indicate that: i) Ca²⁺ efflux is promoted by external acidification, is accompanied by a membrane depolarization, and by an intracellular acidification greater than the one resulting from H_o "leak" and PMCA-mediated ATP hydrolysis; ii) H_o-dependent Ca²⁺ efflux is inhibited by PMCA blockers and by ATP depletion and is accelerated by membrane depolarization (~3 fold by 20 mV depolarization); iii) the coupling ratio of the Ca²⁺/H⁺ exchange depends on H_o: at an extracellular pH (pH_o)=6.5, the ratio is 1Ca²⁺:~3H⁺; at pH_o=8.2, Ca²⁺ efflux rate is 3 times slower and the ratio is 1Ca²⁺: <1H⁺.