The depletion of an inositol 1, 4,5-trisphosphate-sensitive intracellular Ca2+ pool has been proposed to be the signal for Ca2+ entry in agonist-activated cells. Consistent with this idea, thapsigargin, which releases intracellular Ca2+ without inositol phosphate formation, has been reported to activate Ca2+ entry in certain cells. We now report the effects of thapsigargin on Ca2+ entry in parotid acinar cells. In fura-2-loaded parotid acinar cells, thapsigargin caused a sustained elevation of [Ca2+], but did not increase inositol phosphate formation. In the absence of extracellular Ca2+, the increase in [Ca2+], was transient, suggesting that thapsigargin activates both the release of Ca2+ from intracellular stores and the entry of Ca2+ from the extracellular space. In the absence of extracellular Ca2+, pretreatment with methacholine, an agonist believed to mobilize Ca2+ through the production of inositol 1,4,5-trisphosphate, inhibited but did not completely block the response to thapsigargin; likewise, pretreatment with thapsigargin inhibited the response to methacholine. In permeabilized cells, thapsigargin gradually released Ca2+, whereas inositol 1,4,5-trisphosphate caused a rapid and transient discharge of Ca2+. The simultaneous addition of thapsigargin with inositol 1,4,5-trisphosphate evoked a maximum Ca2+ release similar to that for inositol 1,4,5-trisphosphate alone, but the reuptake seen with inositol 1,4,5-trisphosphate alone was abolished. In intact cells, methacholine and thapsigargin together produced a greater initial release of Ca2+ than either alone, but they were not additive in the sustained phase of Ca2+ mobilization. These results demonstrate that the mechanisms for activation of Ca2+ entry by thapsigargin and methacholine are the same and are consistent with the idea that entry is initiated by the depletion of the intracellular inositol 1,4,5-trisphosphate-sensitive Ca2+ pool. The results also indicate that, in contrast to previously proposed models, Ca2+ entry into agonist-activated cells occurs directly across the plasma membrane to the cytoplasm rather than through a cycle of uptake and release by the intracellular Ca2+ pool.