1. Intracellular recording of the transmembrane potential in mouse pancreatic cells revealed a membrane potential of -20.1 +/- 0.8 mV for islet cells and -41.2 +/- 1.4 mV for acinar cells.2. The membrane potential of islet cells was glucose dependent and in the absence of glucose the cells hyperpolarized to -32.7 mV; with glucose 27.7 mM they depolarized to -16.1 mV.3. Above a threshold concentration of glucose (4 mM) small action potentials of amplitude 1-4 mV were induced in islet cells. The percentage of cells impaled exhibiting action potentials reached a maximum of 80% at 27.7 mM glucose.4. Mannose 16.6 mM was similar to glucose in its ability to induce action potential discharge in islet cells.5. 2,4-Dinitrophenol (0.25 mM) hyperpolarized islet cells and blocked electrical activity induced by glucose 11.1 mM.6. Adrenaline (1 muM) completely blocked glucose-induced electrical activity but without altering the membrane potential.7. The origin and functional significance of glucose-induced electrical activity in islet cells is discussed in relation to insulin secretion.