The recent availability of activators of the mitochondrial Ca(2+) uniporter allows direct testing of the influence of mitochondrial Ca(2+) uptake on the overall Ca(2+) homeostasis of the cell. We show here that activation of mitochondrial Ca(2+) uptake by 4,4',4''-(4-propyl-[1H]-pyrazole-1,3,5-triyl)trisphenol (PPT) or kaempferol stimulates histamine-induced Ca(2+) release from the endoplasmic reticulum (ER) and that this effect is enhanced if the mitochondrial Na(+)-Ca(2+) exchanger is simultaneously inhibited with CGP37157. This suggests that both Ca(2+) uptake and release from mitochondria control the ability of local Ca(2+) microdomains to produce feedback inhibition of inositol 1,4,5-trisphosphate receptors (InsP(3)Rs). In addition, the ability of mitochondria to control Ca(2+) release from the ER allows them to modulate cytosolic Ca(2+) oscillations. In histamine stimulated HeLa cells and human fibroblasts, both PPT and kaempferol initially stimulated and later inhibited oscillations, although kaempferol usually induced a more prolonged period of stimulation. Both compounds were also able to induce the generation of Ca(2+) oscillations in previously silent fibroblasts. Our data suggest that cytosolic Ca(2+) oscillations are exquisitely sensitive to the rates of mitochondrial Ca(2+) uptake and release, which precisely control the size of the local Ca(2+) microdomains around InsP(3)Rs and thus the ability to produce feedback activation or inhibition of Ca(2+) release.