Mitochondria take up and then release Ca²⁺ ions, and this trans-mitochondrial Ca²⁺ cycle modulates plasma membrane Ca²⁺ channels in several ways. By buffering incoming Ca²⁺ ions, subplasmalemmal mitochondria prevent the Ca²⁺-dependent inactivation of Ca²⁺ entry channels whereas, by relaying the captured Ca²⁺ ions to the endoplasmic reticulum, mitochondria facilitate the refilling of intracellular Ca²⁺ stores and promote the closure of store-operated Ca²⁺ channels. Conditions that remove mitochondria from the subplasmalemmal space thus prevent the refilling of the endoplasmic reticulum and might augment cytosolic Ca²⁺ elevations to toxic levels. Mitochondria also move protons to generate ATP, and their metabolic activity is thus affected by changes in cell pH. By measuring pH changes simultaneously in the cytosol and in mitochondria, we observed that both compartments acidify during cytosolic elevations evoked by Ca²⁺-mobilizing agonists. Contrary to a common assumption, the trans-mitochondria pH gradient did not increase, but instead nearly collapsed during these bouts of acidification evoked by agonists. The acid load was proportional to the cumulative increase in cytosolic Ca²⁺, regardless of whether cells were stimulated with Ca²⁺-mobilizing agonists or with thapsigargin to deplete Ca²⁺ stores and activate store-operated Ca²⁺ channels. In contrast, no acidification was observed despite massive Ca²⁺ elevations when plasma membrane Ca²⁺ pumps (PMCA) were inhibited with 5 mM La³⁺ or alkaline pH_o(8.8). This indicates that the excess acid is due to the activity of PMCA, consistent with the thermodynamics of these pumps that mediate the import of two H⁺ ions for each Ca²⁺ ion extruded. The collapse of the mitochondrial pH gradient caused by active PMCA shuts down ATP production when Ca²⁺ is extruded from cells during the restoration of normal Ca²⁺ levels. This pH connection renders mitochondria functionally silent, uncoupling metabolism from cell signalling during the decaying phase of Ca²⁺ responses.