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Potassium channels, especially small and intermediate conductance KCa channels, have important roles in controlling cell proliferation and differentiation. Thus far regulation of these channels is reported to be primarily at the level of expression, in response to activation of the central growth regulatory signaling pathway (i.e., growth factor receptor tyrosine kinase/Ras/Raf/MEK/ERK). Therefore, the function and regulation of these cell growth-associated channels must be viewed differently from that of channels which govern electrical signaling in excitable cells, and which are typically studied in terms of their transient modulation by G-protein coupled receptors. Although there are suggestions that potassium channels also contribute to growth regulation in excitable cells, a coherent picture of this role in these systems is still emerging. For fibroblasts and T-lymphocytes, it is clear that growth factor and oncogenic upregulation of a unique KCa channel (or possibly KCa channel class) is stimulatory for cell proliferation and activation, respectively. This mitogenic channel has a single channel conductance in the range of 33–39 pS, it is charybdotoxin-sensitive and apamin-insensitive, and its gating is voltage-independent. Recent cloning data suggest that the KCa channel (or channel class) described for fibroblasts and T-cells has a widespread distribution in other mitogenically active (non-neuronal) tissues. A number of studies are now underway to understand the physiology, pharmacology and regulation of this channel. Further, it is now critical to determine how KCa activity integrates into the signaling pathways which convey growth regulatory information from the cell membrane, to the nucleus, and then to the ultimate effectors for cell proliferation or differentiation. It has also become apparent that these growth regulatory signaling systems interact with other channel types, affecting channel densities at the level of expression, and channel activities at the level of gating modulation. Therefore, it seems most appropriate to view ion channel function in the context of how it contributes to and is affected by both cell growth activity, and the biochemical signaling systems linked to growth control. [ABSTRACT FROM AUTHOR]