Glucose-induced membrane electrical activity was recorded from single isolated mouse islets of Langerhans exposed to steady levels or to step changes of glucose concentration. Superimposed on the well-known rapid (approximately 15-second period) alternations in membrane potential, slow oscillations in the intensity of the electrical activity were observed having a period of 4.6 +/- 0.2 minute (mean +/- SEM, n = 19 islets) and a range of 3.0-6.2 minutes. The largest observed amplitude of oscillation was nearly 50% of the mean intensity. In 62 consecutive recordings from different islets, eleven (18%) islets oscillated steadily after 10 minutes of constant bath conditions while an additional eight islets (13%) oscillated only transiently following abrupt changes of islet stimulation. The oscillations, when triggered by changes of islet stimulation, appeared to be a "ringing" of the biphasic kinetics previously described for both electrical activity and insulin release. Since glucose-induced electrical activity is known to be well correlated with insulin secretory rate, these observations suggest that single isolated mouse islets may also display periodic insulin secretion.