Similar to growth factors aldosterone stimulates Na+/H + exchange in renal target cells leading to cytoplasmic alkalinization. An alkaline intracellular pH reduces the H + bonds between repressor proteins and DNA leading to the destabilization of the nuclear chromatin. We observed that sustained alkaline stress "per se" can lead to malignant transformation of Madin-Darby canine kidney (MDCK) cells. Cells grown for two weeks in alkaline culture medium (pH 7.8) developed multiple "foci" composed of spindle-shaped pleomorphic cells lacking contact inhibition and exhibiting poor adhesion to the culture support, typical characteristics of dedifferentiated tumor cells. "Focus" cells were cloned and grown in standard medium (pH 7.4). Cells maintained their abnormal growth pattern, indicating stable pH-induced genetic transformation. Cells were fused with polyethylene glycol to giant cells and impaled with microelectrodes. In contrast to non-transformed giant MDCK ceils the plasma membrane potential showed spontaneous oscillations that could be virtually abolished by the omission of extracellular Ca 2+ or by the addition of the K + channel blocker Ba 2+. We conclude that sustained alkaline stress can induce malignant transformation in MDCK cells indicated by an abnormal growth pattern and by membrane potential oscillations most likely due to Ca 2+ activated K + channels in the plasma membrane.