Insulin secretion in normal B-cells is pulsatile, a consequence of oscillations in the cell membrane potential (MP) and cytosolic calcium activity ([Ca2+](c)). We simultaneously monitored glucose-induced changes in [Ca2+](c) and in the mitochondrial membrane potential Delta Psi, as a measure for ATP generation. Increasing the glucose concentration hom 0.5 to 15 mM led to the well-known hyperpolarization of Delta Psi and ATP-dependent lowering of [Ca2+](c). However, as soon as [Ca2+](c) rose due to the opening of voltage-dependent Ca2+ channels, Delta Psi depolarized and thereafter oscillations in [Ca2+](c) were parallel to oscillations in Delta Psi. A depolarization or oscillations of Delta Psi cannot be evoked by a substimulatory glucose concentration, but Ca2+ influx provoked by 30 mM KCl was followed by a depolarization of Delta Psi. The following feedback loop is suggested: Glucose metabolism via mitochondrial ATP production and closure of K-ATP(+) channels induces an increase in [Ca2+](c). The rise in [Ca2+](c) in turn decreases ATP synthesis by depolarizing Delta Psi, thus transiently terminating Ca2+ influx.