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+ (Ho); 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 Ho, intracellular ATP, PMCA blockers, and membrane potential (VM) 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 Ho "leak" and PMCA-mediated ATP hydrolysis; ii) Ho-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 Ho: at an extracellular pH (pHo)=6.5, the ratio is 1Ca2+:~3H+; at pHo=8.2, Ca2+ efflux rate is 3 times slower and the ratio is 1Ca2+: <1H+.