The fluorescent Ca²⁺ indicator Fura 2 was used to monitor Ca²⁺ release induced by the Ins(1,4,5)P₃-mobilizing agonist angiotensin II (Ag II), caffeine and 2,5-di-(tert-butyl)-1,4-benzohydroquinone (tuBHQ), in intact bovine adrenal chromaffin cells. Under low external Ca²⁺ conditions, tuBHQ, Ag II and caffeine elicited Ca²⁺ rises, indicating Ca²⁺ release from internal stores. Prior addition of Ag II had no noticeable effect on the extent of release of Ca²⁺ induced by tuBHQ. Stimulation of the cells with tuBHQ before either Ag II or caffeine, similarly had no effect on Ca²⁺ released by these two agonists. It was concluded, therefore, that there is a third intracellular Ca²⁺ store in bovine adrenal chromaffin cells, distinct and non-overlapping, from those sensitive to caffeine or Ins(1,4,5)P₃-Mobilizing agonists.