The extent to which inositol 1,4,5-trisphosphate (InsP(3))-induced calcium signals are localized is a critical parameter for understanding the mechanism of effector activation, The spatial characteristics of InsP(3)-mediated calcium signals were determined by targeting a dextran-based calcium indicator to intracellular membranes through the in situ addition of a geranylgeranyl lipid group. Elementary calcium-release events observed with this indicator typically lasted less than 33 milliseconds, had diameters less than 2 micrometers, and were uncoupled from each other by the calcium buffer EGTA. Cellwide calcium transients are likely to result from synchronized triggering of such local release events, suggesting that calcium-dependent effector proteins could be selectively activated by localization near sites of local calcium release.