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Changes in the plasma membrane potential of two histanmine-releasing cells, rat peritoneal mast cells and basophilic leukemia cells 2H3 (RBL 2H3), were recorded with the potential-sensitive dye bis-oxonol. For mast cells, the presence of ouabain or the absence of K +increased the fluorescence intensity of bis-oxonol; gramicidin had no effect. For RBL 2H3 cells, the presence of ouabain and the absence of K +also increased bis-oxonol fluorescence but gramicidin also increased it. These results show that the plasma membrane potential of RBL 2H3 cells is set, in part, by the acitivity of the Na +pump and in part by the K +conductance, while that of rat mast cells is set predominantly by the Na +pump.