1. Intracellular micro-electrode recording techniques have been used to study the effects of varying the external ion concentration on the membrane potential and glucose-induced electrical activity in cells from mouse islets of Langerhans.2. Increasing [K](o) to 47 mM depolarized islet cells without inducing electrical activity. Normal action potentials were generated in response to glucose after removal of [K](o) for 60 min.3. Reduction of [Cl](o) to 12 mM did not affect the membrane potential and glucose still induced electrical activity.4. Reduction of [Na](o) to 26 mM increased the amplitude of action potentials; subsequently the cells depolarized.5. Removal of [Ca](o) caused cells, already firing action potentials intermittently in response to glucose, to change their pattern of discharge to one of continuous firing. The time constant of the action potentials was also increased. Depletion of calcium for 60 min before addition of glucose prevented the appearance of electrical activity.6. Increasing [Ca](o) threefold to 7.7 mM in the presence of glucose, 11.1 mM, increased action potential amplitude to 12 mV, but a tenfold increase of [Ca](o) to 25.6 mM completely blocked action potential discharge.7. Exposure of islet cells simultaneously to reduced [Na](o) (26 mM) and increased [Ca](o) (7.7 mM) increased the amplitude of glucose-induced action potentials to about 20 mV.8. Strontium, 2.56 mM, was an effective substitute for [Ca](o), 2.56 mM, and maintained a normal pattern of electrical activity in response to glucose.9. Increasing the magnesium concentration tenfold to 11.3 mM did not block electrical activity whereas manganese, 2 mM, blocked glucose-induced action potentials and depolarized islet cells.10. It was concluded that the action potential induced by glucose in islet beta-cells is due predominantly to calcium entry and that sodium ions tend to repress this calcium influx.