A sarcolemma-enriched membrane fraction (SL) was prepared from the hearts of Sprague-Dawley rats and its ability to bind Ca++ was measured by equilibrium dialysis. We found that the effect of taurine on SL Ca++ binding varied with the buffer and with Na+ concentration. In Tris, in the presence of Na+ (140 mM), taurine (10 mM) increased the affinity but decreased the maximal binding of Ca++ (0.5-7 mM). In the absence of Na+, taurine decreased the affinity without altering the maximal binding. These effects on Ca++ binding were absent in bicarbonate or Krebs-Henseleit buffers. However, incubations with A23187, a Ca++ ionophore, and lanthanum, a Ca++ antagonist, indicated that SL membranes incubated in Tris, but not in buffers containing bicarbonate, were sealed vesicles with internal environments low in Ca++. High-affinity binding of Ca++ (10(-6)-10(-4) M) was measured in modified Krebs-Henseleit buffers. Taurine decreased Ca++ binding in a high-Na+ (145 mM), low-K+ (4.7 mM) buffer. Taurine increased Ca++ binding in both 4.7 mM Na+-145 mM K+ and 25 mM Na+-4.7 mM K+ buffers. Taurine also increased Ca++ binding in the presence of ATP. Thus, taurine increased high-affinity Ca++ binding in "intracellular" buffers, but it did not affect low-affinity Ca++ binding in "extracellular" buffers. These results suggest taurine may exert its cardiotonic actions through modulation of the high-affinity Ca++ binding sites on the internal aspect of the SL.