The membrane potential of mouse pancreatic B cells was recorded with microelectrodes. In certain cells, both the slow waves of depolarization and the intervals of repolarization triggered by glucose (10 or 15 mM) displayed regular oscillations in their duration, though the concentration of the sugar remained constant. When forskolin (0.2 microM), an activator of adenylate cyclase, was added to the medium, the electrical activity rapidly became very regular, with slow waves and intervals of constant duration. This effect was unrelated to the overall increase in activity also brought about by forskolin. The oscillations resumed in 75% of the cells after withdrawal of the drug. Under similar conditions, forskolin rapidly and reversibly raised the cAMP concentration in the islets. The data suggest that cAMP is an important modulator of the electrical activity triggered by glucose in insulin-secreting cells.