In non-excitable cells, one major route for Ca²⁺ influx is through store-operated Ca²⁺ channels (SOC). These channels are activated by emptying intracellular Ca²⁺ stores and like voltage-operated Ca²⁺ channels, SOC channels become permeable to monovalent cations under extracellular divalent free conditions. Using the whole-cell patch-clamp technique, we characterized the permeation of monovalent cations in the rat basophilic leukemia (RBL-2h3m1) cell line. The Ca²⁺ stores were depleted with intracellular IP₃ to activate SOC channels and 2 mM internal Mg²⁺ were used to prevent the activation of MIC-channels. The external standard solution contained 115 mM Na⁺, 5 mM K⁺, 2 mM Mg²⁺ and 2 mM Ca²⁺. After the removal of external Ca²⁺ we observed a rapid development of a large inward current (2.6 ±0.6 pA/pF at -80 mV). Additional experiments in which Na⁺ was replaced by NMDG⁺ indicated that the inward current was carried by Na⁺. Furthermore we found that Li⁺ and Cs⁺ also were able to support inward currents in these cells. Since, Mg²⁺ was always present in the external solution, the reported inward rectifying current appears to be carried through ionic channels which are different from SOC channels.