Stimulation of animal cells with a number of agonists causes a rise in intracellular Ca 2+ which can occur at least temporarily even in the absence of any extracellular Ca 2+ . The explanation for this phenomenon lies in the mobilisation of Ca 2+ from intracellular stores, and a recent consensus of opinion now focuses on the endoplasmic reticulum as the principal organella involved. The mitochandrion probably does not contribute any mobilized Ca 2+ , but instead increases its Ca 2+ content in response to the cytosolic rise. The endoplasmic reticular (e.r.) Ca 2+ pool which is responsive to agonists, is only a part of the total e.r. Ca 2+ , and this particular pool may well be closely associated with the plasma membrane.The compound which carries the message to mobilise Ca 2+ from the receptor in the plasma membrane to the e r is inositol (1,4,5)trisphosphatens(1,4,5) P3 After it has interacted with a specific receptor in the e.r.to release Ca 2+ ,Ins(1,4,5) P3 is deactivated by a specific 5-phosphatase which converts it to inositol(1,4)bisphosphate.