To better understand the functional role of the mitochondrial network in shaping the Ca2+ signals in living cells, we took advantage both of the newest genetically engineered green fluorescent protein-based Ca2+ sensors ("Cameleons," "Camgaroos," and "Pericams") and of the classical Ca(2+)-sensitive photoprotein aequorin, all targeted to the mitochondrial matrix. The properties of the green fluorescent protein-based probes in terms of subcellular localization, photosensitivity, and Ca2+ affinity have been analyzed in detail. It is concluded that the ratiometric pericam is, at present, the most reliable mitochondrial Ca2+ probe for single cell studies, although this probe too is not devoid of problems. The results obtained with ratiometric pericam in single cells, combined with those obtained at the population level with aequorin, provide strong evidence demonstrating that the close vicinity of mitochondria to the Ca2+ release channels (and thus responsible for the fast uptake of Ca2+ by mitochondria upon receptor activation) are highly stable in time, suggesting the existence of specific interactions between mitochondria and the endoplasmic reticulum.