Ca²⁺ binding proteins (CaBPs) show cell- specific expression and are present in many different cell types, yet the physiological basis underlying their selective distribution patterns remains unclear. We used confocal line-scan microscopy together with photo- release of IP₃ in Xenopus oocytes to investigate the actions of mobile cytosolic CaBPs on the spatio-temporal properties of IP₃-evoked Ca²⁺ signals. Parvalbumin (PV), a CaBP with slow Ca²⁺-binding kinetics, shortened the duration of IP₃-evoked Ca²⁺ signals and ?balkanized? global responses into discrete localized events (puffs). In contrast, calretinin (CR), a presumed fast buffer, prolonged Ca²⁺ responses and promoted ?globalization? of spatially uniform Ca²⁺ signals at high [IP₃]. Oocytes loaded with CR or PV showed Ca²⁺ puffs following photolysis flashes that were sub-threshold in controls, and the spatio-temporal properties of these localized events were differentially modulated by PV and CR. In comparison to results we previously obtained with exogenous Ca²⁺ buffers, PV closely mimicked the actions of the slow buffer EGTA, whereas CR showed important differences from the fast buffer BAPTA. Most notably, puffs were never observed after loading BAPTA, and this exogenous buffer did not show the marked sensitization of IP₃ action evident with CR. The ability of Ca²⁺ buffers and CaBPs with differing kinetics to fine-tune both global and local intracellular Ca²⁺ signals is likely to have significant physiological implications.