Calcium binding and Na-Ca exchange activity were measured in isolated cardiac plasma membrane vesicles under various ionic conditions. A model was developed to describe the Ca binding characteristics of cardiac sarcolemmal vesicles using the Gouy-Chapman theory of the diffuse double layer with specific cation binding to phospholipid carboxyl and phosphate groups. The surface association constants used for Ca, Na, K and H binding to both of these groups were 7,0.63, 0.3 and 3800 M-1, respectively. This model allows the estimation of surface [Ca] under any specific ionic conditions. The effects of the divalent screening cation, dimethonium, on Ca binding and Na-Ca exchange were compared. Dimethonium had no significant effect on Ca binding at high ionic strength (150 mM KCl), but strongly depressed Ca binding at low ionic strength. Dimethonium had no significant effect on Na-Ca exchange (Na-inside dependent Ca influx) at either high or low ionic strength. These results suggest that the Ca sites of the Na-Ca exchanger are in a physical environment where they are either not exposed to or not sensitive to surface [Ca].