In the last years, increasing evidence has been accumulated for an important role of mitochondrial Ca²⁺-uptake in the modulation of cytosolic [Ca²⁺] homeostasis. Rapid Ca²⁺-uptake by mitochondria controls the size of the local [Ca²⁺] microdomains created by the opening of Ca²⁺ channels both in the plasma membrane and in the endoplasmic reticulum (ER). In this way, mitochondria may modulate many physiological phenomena triggered by cytosolic high-Ca²⁺ microdomains, such as neurotransmitter secretion. Moreover, many Ca²⁺ channels are feedback modulated by the local [Ca²⁺] in the cytosolic mouth of the channel. Mitochondria placed close to these channels may thus modulate them by taking up Ca²⁺ and reducing the size of those local [Ca²⁺] microdomains. We have investigated the effects of changes in mitochondrial Ca²⁺-fluxes on both the rate of ER-Ca²⁺-release through inositol 1,4,5-trisphosphate receptors (InsP3R) and the development of regenerative InsP3-mediated cytosolic [Ca²⁺] oscillations. We show that both activation of mitochondrial Ca²⁺-uptake and inhibition of mitochondrial Ca²⁺-release activated InsP3-induced ER-Ca²⁺-release and oscillations. On the contrary, inhibition of mitochondrial Ca²⁺-uptake reduced InsP3-induced Ca²⁺-release and blocked oscillations. These findings are consistent with the idea of mitochondria controlling the size of the local [Ca²⁺] microdomain surrounding InsP3R which is responsible of feedback-Ca²⁺-inhibition of Ca²⁺-release. Blocking mitochondrial Ca²⁺-uptake thus enhances inhibition of InsP3 receptors and the contrary occurs when mitochondrial Ca²⁺-uptake is activated or mitochondrial Ca²⁺-release is inhibited. In conclusion, mitochondria are key controllers of Ca²⁺-release through InsP3R and hence of dynamic Ca²⁺ homeostatic phenomena such as oscillations.