Dear Ian As with the UV-probe for sodium there is preliminary indication that the spectral characterisitcs of the probe differe markedly within the cell environment from those expected from in vitro information. With SBFI there is a marked shift in the wavelength dependencies, so much so that in some cell types (including cardiac cells and renal tubules) at the lower UV-range where you would expect fluorescence increases with Na increases you see no fluorescence change, while the upper UV range shows significant fluorescence decreases at the same time, hence allowing ratiometric analyses. You would not expect this from in vitro spectral data. Something equally weird seems to occur with Sodium green such that given the way you would look at it on the MRC-600 you may not see anything. Sounds like its "Back to the cuvette and fluorimeter" to work out what is happening in your loaded cells. The dye certainly loads into cardiac cells, but resting images of renal tubules look much "brighter" and I'm sure that a lot of this is due to a much higher resting [Na+]. As a start scan your loaded cells and then lyse them and look at what comes out in the spec. My guesss is the 488nm/530nm combination is less than ideal for intracellular Sodium green. Cheers, David. PS. Hope its going well in the US otherwise.