Ca2+ and Mg2+ binding to rat parvalbumin was measured by means of the fluorescent Ca2+ indicator fluo-3 using a method developed earlier [Eberhard, M. & Erne, P. (1991) Eur. J. Biochem. 202, 1333-1338]. We demonstrate that rat parvalbumin contains two equivalent Ca2+/Mg2+ binding sites and that Ca2+ and Mg2+ compete for the same sites. Dissociation constants (Kd) for Ca2+ and Mg2+ in Hepes buffer containing 150 mM K+ at 35 degrees C and pH 7.2 are 11.0 +/- 1.8 nM and 41 +/- 8 microM, respectively. At an ionic strength below 0.2 M, Kd values of Ca2+ binding to rat parvalbumin are approximately proportional to the ion concentration. Kd values of Ca2+ binding were found to be about fourfold larger in the presence of Na+ as compared with K+, indicating that Na+ distinctly influences Ca2+ binding to rat parvalbumin. Both Ca2+ and Mg2+ binding to parvalbumin are exothermic whereas Ca2+ and Mg2+ binding to fluo-3 are endothermic entropy-driven processes.