Ins P 3 has two important functions in generating Ca 2+ oscillations. It releases Ca 2+ from the internal store and it can contribute to Ca 2+ entry. A hypothesis has been developed to describe a mechanism for Ca 2+ oscillations with particular emphasis on the way agonist concentration regulates oscillator frequency. The main idea is that the Ins P 3 receptors are sensitized to release Ca 2+ periodically by cyclical fluctuations of Ca 2+ within the lumen of the endoplasmic reticulum. Each time a pulse of Ca 2+ is released, the luminal level of Ca 2+ declines and has to be replenished before the Ins P 3 receptors are resensitized to deliver the next pulse of Ca 2+ . It is this loading of the internal store that explains why frequency is sensitive to external Ca 2+ and may also account for how variations in agonist concentration are translated into changes in oscillation frequency. Variations in agonist-induced entry of external Ca 2+ , which can occur through different mechanisms, determine the variable rates of store loading responsible for adjusting the sensitivity of the Ins P 3 receptors to produce the periodic pulses of Ca 2+ . The Ca 2+ oscillator is an effective analogue-to-digital converter in that variations in the concentration of the external stimulus are translated into a change in oscillator frequency.