Many cell types show repetitive short lasting cytosolic calcium spikes with long interspike periods when stimulated with submaximal concentrations of agonists linked to the phosphoinositide signaling pathway. In pancreatic acinar cells these spikes have been shown to be evoked by constant levels of inositol trisphosphate through a mechanism of calcium-induced calcium release and do not depend acutely on the presence of external calcium. However, the processes involved in the interspike period have remained unclear. Here we report that the endoplasmic reticulum Ca(2+)-ATPase play a significant role, not only in resequestering calcium after a spike, but also in regulating the long interspike period. Decreasing the activity of the endoplasmic reticulum calcium pumps leads to shorter interspike intervals and thus higher spiking frequencies, while the duration of each spike increases. The endoplasmic reticulum Ca(2+)-ATPases are able to entirely suppress a response that can subsequently be evoked by partial inhibition of the pumps. This suggests that during the interspike period there is a considerable amount of calcium released from intracellular stores, which is rapidly buffered by the endoplasmic reticulum calcium pumps and the cytosolic calcium-binding proteins. A calcium spike will be initiated by calcium-induced calcium release only when the buffering is saturated.