Changes in the concentration of free calcium ions, in the cytoplasm, [Ca2+]j, are used to trigger or regulate a wide variety of cellular functions. Methods for measuring the [Ca2+}i are thus important tools for investigating cell activation. Two factors make these measurements difficult. Compared with the free concentrations of the potentially interfering cations, LFF, Mg2, Na and K, [Ca2]i is very low being around 10' MM at rest and rising perhaps to lO M on maximum activation. The detection

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... stems thus need excellent sensitivity and selectivity. Secondly, cells are very small so that considerable ingenuity or manual skill is needed to introduce the detection system into intact cells without, wrecking them. The two main approaches have been either to make calcium-selective microelectrodes small and sharp enough to impale single cells, or to introduce into cytoplasm, by various means, indicators which give an optical signal, absorbance, fluorescence or intrinsic luminescence dependent on, and thus reporting [Ca2+li. This brief account focuses mainly on microelectrodes and a newly developed fluorescent indicator that can be incorporated non-disruptively into populations of cells of any size.