IN many cell types, receptor-mediated Ca²⁺ release from internal stores is followed by Ca²⁺ influx across the plasma membrane^1–3. The sustained entry of Ca²⁺ is thought to result partly from the depletion of intracellular Ca²⁺ pools^4,5. Most investigations have characterized Ca²⁺ influx indirectly by measuring Ca²⁺-activated currents^6–9 or using Fura-2 quenching by Mn², which in some cells enters the cells by the same influx pathway^10,11. But only a few studies have investigated this Ca²⁺ entry pathway more directly^12–14. We have combined patch-clamp and Fura-2 measurements to monitor membrane currents in mast cells under conditions where intracellular Ca²⁺ stores were emptied by either inositol 1,4,5-trisphosphate, ionomycin, or excess of the Ca²⁺ chelator EGTA. The depletion of Ca²⁺ pools by these independent mechanisms commonly induced activation of a sustained calcium inward current that was highly selective for Ca²⁺ ions over Ba²⁺, Sr²⁺ and Mn²⁺. This Ca²⁺ current, which we termI_CRAC (calcium release-activated calcium), is not voltage-activated and shows a characteristic inward rectification. It may be the mechanism by which electrically nonexcitable cells maintain raised intracellular Ca²⁺ concentrations and replenish their empty Ca²⁺ stores after receptor stimulation.