In pancreatic β cells, the increase in the ATP/ADP ratio that follows a stimulation by glucose is thought to play an important role in the Ca²⁺-dependent increase in insulin secretion. Here we have investigated the possible interactions between Ca²⁺ and adenine nucleotides in mouse islets. Measurements of both parameters in the same single islet showed that the rise in the ATP/ADP ratio precedes any rise in the cytoplasmic free-Ca²⁺ concentration ([Ca²⁺]_i) and is already present during the initial transient lowering of [Ca²⁺]_i produced by the sugar. Blockade of Ca²⁺ influx with nimodipine did not prevent the concentration-dependent increase in the ATP/ADP ratio produced by glucose and even augmented the ratio at all glucose concentrations which normally stimulate Ca²⁺ influx. In contrast, stimulation of Ca²⁺ influx by 30 mM K⁺ or 100 µM tolbutamide lowered the ATP/ADP ratio. This lowering was of rapid onset and reversibility, sustained and prevented by nimodipine or omission of extracellular Ca²⁺. It was, however, not attenuated after blockade of secretion by activation of α₂-adrenoceptors. The difference in islet ATP/ADP ratio during blockade and stimulation of Ca²⁺ influx was similar to that observed between threshold and submaximal glucose concentrations. The results suggest that the following feedback loop could control the oscillations of membrane potential and [Ca²⁺]_i in β cells. Glucose metabolism increases the ATP/ADP ratio in a Ca²⁺-independent manner, which leads to closure of ATP-sensitive K⁺ channels, depolarization and stimulation of Ca²⁺ influx. The resulting increase in [Ca²⁺]_i causes a larger consumption than production of ATP, which induces reopening of ATP-sensitive K⁺ channels and arrest of Ca²⁺ influx. Upon lowering of [Ca²⁺]_i the ATP/ADP ratio increases again and a new cycle may start.