Thus far, the methods used to determine erythrocyte Ca 2 + influx have not allowed the assessment of the kinetics of ion uptake. To overcome this drawback, we studied a new method, using the fluorescent Ca 2 + -chelator fura-2, which directly quantifies intracellular Ca 2 + changes in human erythrocytes. This method has the advantage over previous techniques that it monitors continuously cellular Ca 2 + levels. The Ca 2 + influx is modulated by cellular membrane potential in the presence of a transmembrane Ca 2 + concentration gradient and exhibits a first slow increase of the intracellular Ca 2 + concentration, followed, after the reachment of a threshold value of 125 +/- 13 nM Ca 2 + , by a faster increase until a plateau is reached. The influx rate is inhibited by dihydropyridines in the micromolar range. These findings support the hypothesis that erythrocyte Ca 2 + influx is mediated by a carrier similar to the slow Ca 2 + channels and is dependent on membrane depolarization.