Numerous hormones and neurotransmitters activate cells by increasing cytosolic calcium concentration ([Ca(2+)](i)), a key regulatory factor for many cellular processes. A pivotal feature of these Ca(2+) signals is the release of Ca(2+) from intracellular stores, which is followed by activation of extracellular calcium influx, allowing refilling of the stores by SERCA pumps associated with the endoplasmic reticulum. Although the mechanisms of calcium release and calcium influx have been extensively studied, the biology of the Ca(2+) stores is poorly understood. The presence of heterogeneous calcium pools in cells has been previously reported [1] [2] [3]. Although recent technical improvements have confirmed this heterogeneity [4], knowledge about the mechanisms underlying Ca(2+) transport within the stores is very scarce and rather speculative. A recent study in polarized exocrine cells [5] has revealed the existence of Ca(2+) tunneling from basolateral stores to luminal pools, where Ca(2+) is initially released upon cell activation. Here, we present evidence that, during stimulation, Ca(2+) transported into basolateral stores by SERCA pumps is conveyed toward the luminal pools driven by proton gradients generated by vacuolar H(+)-ATPases. This finding unveils a new aspect of the machinery of Ca(2+) stores.