Background: Fluorescent indicators that show alterations in excitation and/or emission spectra in response to changes in Ca 2+ are widely used for quantitative cytosolic Ca 2+ measurements. There are several reports of changes in apparent Ca 2+ due only to illumination, however. These results have been attributed either to photodamage to the cells or to photodegradation of the indicator. Light-induced alteration in the behavior of the dye or cells would severely hamper the interpretation of experimental data. We examined this phenomenon in indo-1 loaded cells using confocal laser scanning microscopy. Results: Illumination of indo-1 loaded GH3 cells leads to a decrease in apparent basal Ca 2+ and decreased peaks after depolarization with KCI. When cells were double loaded with indo-1 and fluo-3, the effect of UV illumination was noticed only with the former dye. UV irradiation of indo-1 in simple buffers caused overall photobleaching and conversion to a fluorescent but Ca 2+-insensitive species. The latter effect cannot be canceled by ratiometric calibration and is due to loss of carboxymethyl groups from the anilino nitrogens. This photodegradation was inhibited by extracellular administration of 10 to 100 μM Trolox, a water-soluble vitamin E analog. Conclusions: Photodegradation processes like that observed for indo-1 are likely to be possible for all cation indicators that contain bis(carboxymethyl)anilino moieties, which include essentially all fluorescent indicators for Ca 2+ and Mg 2+ currently in biological use. If unrecognized, this photochemical dealkylation leads to an underestimation of the analyte concentrations depending on the intensity and duration of illumination. The problem can be avoided by including cell-permeant antioxidants such as Trolox in the bathing solution. The ultimate solution would be to redesign the indicators to minimize photodegradation in the absence of antioxidants.